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My analysis of Tuchulcha's motives based on canon only.
Why does this allmighty entity help the Lion? What does Tuchulcha get from their alliance? Let's sum it up.
At the first glance the answer to the first question is pretty simple. Tuchulcha wants to travel to Caliban with the Lion and is willing to help him in exchange. A simple desire of a simple-minded creature. Or no? This wish isn't a lie - in 'Ruinstorm' Tuchulcha says that it has never lied to the Lion - but dig a bit deeper, and turns out that its answer is only a tip of the iceberg.
First of all, during the Heresy Tuchulcha was able to teleport in almost any place in the galaxy and was limited only by the strongest manifestations of Chaos powers such as the Ruinstorm. And if - in theory - Tuchulcha couldn't perform the particular jump to Caliban, there was Typhon who desired to have it by his side. Persumably, if told, he would willingly deliver Tuchulcha to Caliban on his 'Terminus Ext'.
But Tuchulcha didn't want the alliance with Typhon. Instead Tuchulcha waited for the return of the Lion for many years. Tuchulcha needed the Lion - no other leaving creature. But what could the Lion offer which wasn't available for any other individual? Not firepower or a legion - something that Tuchulcha could use.
The only thing that comes to mind is the Lion's alliance with Watchers in the Dark. Presumabely, if one becomes ally of the Lion, Watchers would have to stay neutral toward this creature (or try to change the Lion's mind). Tuchulcha proves to the Lion that it can be useful for the him and gets its ticket abroad 'Invincible Reason'. The ticket to Caliban. This is what The Lion possibly is in Tuchulcha's metaphorical eyes.
Tuchulcha's behavior may seem childish but this creature isn't silly at all. Tuchulcha from the begining is able to read the mind of the Lion (and a device as powerful as Pharos Beacon is unable to do it). So Tuchulcha is aware of impression it made on the Lion.
'The Lion was dumbfounded, unable to comprehend what had happened for several seconds. He eventually realised that Tuchulcha must have moved the battle-barge into the warp and displaced itself, the primarch and tech-priest onto the vessel an instant later. Part of the Lion was appalled by the dangerous situation and Iaxis’s naiveté in allowing this to happen; a greater part of him marvelled at the unprecedented power on display.' - 'The Lion', Gav Thrope
From this moment Tuchulcha knows that the Lion admires its power and wants to use it - and will allow to join him.
Secondly, why does Tuchulcha want to get to Caliban? And why should someone stop it from coming there? Tuchulcha desires to get to Caliban so much that it calls a 'savior' a creature who can grant its wish.
‘I have been waiting a long time for the Lion to return. I saw him, the first time he came, and knew that my saviour had been delivered to me. The Perditians trapped me here, but with the aid of Iaxis I have been able to loose my bonds. I have remained solely because I knew the Lion would return to me.’ - 'The Lion', Gav Thrope
To figure it let's check what Tuchulcha is and what its potential allies think about it.
1) The Lion states that Tuchulcha is dangerous sophisticated machine. Tuchulcha isn't lying to him - as always - but when asked directly it gives a very vague responce.
‘I asked what you are, not who you are. Curse you, what am I saying? You are a machine, a sophisticated machine and nothing more.’
'I am everything, Lion. Everywhere. I was once Servant of the Deadly Seas. Now I am the Friend of the Mechanicum.’
‘You are dangerous,’ said the Lion. ‘A war is being waged for possession of you. I should destroy you and save much turmoil and bloodshed.’
‘You cannot destroy me, Lion. Not physically, nor do you desire it. All things desire to possess me. The one they call Typhon dreams much about me. The mind of the other, Midoa, is closed to me. It contains too much iron for my liking. You… You are neither open nor closed. You scare me, Lion. It was not until you came that I knew what fear was. Your return scares me, Lion. I do not wish to be destroyed.’ - 'The Lion', Gav Thrope
Tuchulcha's words sound like pointless blathering, but, ironically, Tuchulcha does give the Lion some clues about its origins. But the Lion doesn't dig into it and decides to consider Tuchulcha some form of xenotech which he can control if uses it with caution:
'It was a bargain of practicality, Redloss reminded himself as the servitor-puppet tottered over to stand before the Lion. Without Tuchulcha the Dark Angels fleet would never have been able to pierce the ruinstorm and reach Macragge. It was obvious, and the Lion had warned as much, that the warp entity wasserving its own alien needs, but the immediate need outweighed that longer-term risk.
Redloss wasn’t convinced by this reasoning and might even have spoken against the plan had he been at Perditus.
As it was, the deal was done and Tuchulcha had been moved from the Invincible Reason to the Honoured Deeds. There would be consequences – that much was tacitly agreed, but their nature was yet to be revealed.' - 'Angels of Caliban', Gav Thrope
So even if the Lion is aware that Tuchulcha wants to use him, he probably has no idea of its plans. He has no objection to allow it to travel to Caliban with him.
2) Then here comes Typhon who tried to ally with Tuchulcha at least twice. From the begining he looks more informed about essence of Tuchulcha than any other superhuman being. But, as shown in 'Unforgiven', he is also mistaken about it and it costs him his entire plan.
‘Some have tried to destroy me, Lord of Angels. Physically, it is impossible, nor should you desire it. All things desire me. The one they call Typhon covets me, but I am not what he thinks I am. I do not wish to be destroyed.’ - Tuchulcha, 'Unforgiven', Gav Thorpe
During the Heresy Typhon was ready to risk his own life to get Tuchulcha - he even entered into a confrontation with the Lion himself in order to 'get' it.
In 40th millenia Typhon, who is now Typhus, once again came up with a plan which included Tuchulcha. This plan was to merge Tuchulcha with Plagueheart and free Plagueheart. Plagueheart is daemon without any doubts - it is stated by many characters and even Tuchulcha confirms it. Possibly, it's daemon of Nurgle.
'I am not a daemon,’ the servitor mouthpiece replied, almost petulant. ‘I am far greater than some mindless facet of an incomprehensible entity. If you wish to see a daemon, cast your eye out to that abomination close at hand. This physical vessel is incapable of pronouncing its name, but it has been known by many titles by the generations of humans that have worshipped it. The latest was the Plagueheart, in the prayers of the people of Ulthor.’ - Tuchulcha, 'Unforgiven', Gav Thrope
Typhon knew things the Lion and Dark Angels had no idea of. Ironically, it didn't help him. Tuchulcha had pretty different aims.
‘No!’ Typhus howled the word, staggering as if struck. Azrael took two steps but the Chaos lord straightened, the blade of Manreaper held before him. ‘No, it cannot be! We have all been tricked!’
(...)
‘We have been lured here, three parts to make the whole,’ the corrupted Terminator answered. ‘The bridge, the key and the Plagueheart, all in this place at the same time. Not to release the Plagueheart, not to bring back your traitorous kin, nor to save your souls. Something greater awakens, dormant for aeons, awaiting the time when it could be brought back. No! Grandfather, I failed you again! The bridge opens again, denying me.’
(...)
‘I will not be denied again!’ he raged. Typhus lumbered into a run, barrelling towards Azrael. The Supreme Grand Master raised his sword to receive the charge. Teeth gritted, his eyes on the gleaming tip of Manreaper, he watched the scythe blade descend.The world disappeared.
(...)
‘It is done,’ croaked the warp device’s servitor. ‘The plague-lord will no longer interfere. We will be remade.’ - 'Unforgiven', Gav Thrope
Well, Typhus yelling for forgiveness is definitely not a thing one expects of him. Looks like there was a great gap in his knowledge, and he greatly messed up.
So, when Typhon/Typhus understands what he's facing he becomes far less enphusiastic about Tuchulcha.
3) At this point it becomes clear that both the Lion and Typhon didn't understand fully what they were allying or trying to ally. But Tuchulcha can speak for itself if anyone cares to listen carefully:
Tuchulcha’s living dummy laughed again, hands slapping limply together in a parody of clapping.
‘They made us, the one split into three,’ the puppet cackled. ‘The essence of Chaos, refined and shaped. They thought they could tame the warp, use me to dig their tunnels and secret ways hidden from the eyes of the Powers That Rule. They did not know that they made something else. Something far grander.’
‘Who? Who made you?’ Azrael demanded.
‘At the dawn of the galaxy, so far removed from humans they might as well be gods. But even they could not tame the warp, only corral it for moments at a time. But that which creates also devours, and I am the foundation of all that was, is and will be. I am the lens, the bridge, the doorway.’ - 'Unforgiven', Gav Thrope
It's funny but Tuchulcha several times mentions the possible transformation but no one understands it because of its vague phrases.
‘Ignore it,’ Ezekiel said quickly, stepping in front of Cypher. He darted a worried glance at Azrael – an expression the Supreme Grand Master had never seen before on the face of his Chief Librarian. ‘We must find some way to destroy it.’
‘You cannot.’ The servitor let out a dry laugh. ‘If I could be destroyed, I would not exist. If you wish to be rid of me, all you have to do is take me home.’
(...)
‘Salvation.’ The servitor sagged. ‘Release. For all of us. But not Perditus. Home. The world you called Caliban.’ - 'Unforgiven', Gav Thrope
Get rid of Tuchulcha - gain something extremely powerful instead, huh.
Summing all things said Tuchulcha is very powerful in its current state. It's not a daemon (let's trust it - it has no reason to lie), so might be some kind of AI-operator of extremely powerful warp-entity. And nobody knows exactly what happens if Tuchulcha and sister-parts are re-united. Well, nobody exept for...
4) ‘We told him that Chaos could not be defeated, only fought.’
(...)
'It is already too late. The Ouroboros stirs. The Lion continues to employ its power.’
- Watchers in the Dark, 'Dreadwing'
Watchers in the Dark seem to be fully aware of what Tuchulcha is - given their interaction in 'Unforgiven' - and they have methods to control it. They fight against Ouroboros but don't object Tuchulcha hanging on onboard 'Invincible Reason' and being hidden in the Rock later.
This brings some questions about their course of actions during Heresy. It seems that Watchers in the Dark didn't reveal to the Lion what he had invited on his ship. Instead Watchers in the Dark agreed to tolerate Tuchulcha onboard. Theoretically, they should be the first to object Tuchulcha approaching to Caliban. It's not clear why they didn't do that, but, perhaps, Watchers wanted Tuchulcha to help the Lion to finish his personal wars a.k.a. Horus Heresy as soon as possible and then to deliver the Lion to Caliban in time so that he can do something to Ouroboros. Or, perhaps, they just wanted to keep Tuchulcha in their sight. Perhaps, they knew from the start that it couldn't re-unite with other parts. But all of this remains to be seen.
Last but not the least. Did Tuchulcha achieve its purpose? It definitely was abble to arrive to remains of Caliban, but it's unclear was the re-unition successful. The only thing which could answer this question is Tuchulcha itself, but it has no servitor to speak anymore (sorry for this little giggle). At the moment it seems like Tuchulcha didn't achieve it, because no new godlike entities appeared in the universe of Warhammer . But it could be a matter of time. Who knows how many centuries such processes can last?
Tuchulcha was very important ally of the Lion, but, if its words are truth, just a little part of Tuchulcha's power was required for that help. In return, it secretly wanted to be reborn into the enormously powerful entity - not for world domination, just to be one creature again.
And if Tuchulcha's plan succeeded it might has benefited from it much more than The Lion and humanity.
Bonus:
'He seeks to possess me, as all others have before,’ replied Tuchulcha. ‘Fear not for me, brave Typhon. You must fulfil your own destiny. Your primarch awaits you. It would be such a waste for you to be slain here. Here, let me help you.’ -Tuchulcha, 'The Lion', Gav Thrope
So this creature is basically saying 'Your primarch waits for you to corrupt him and turn to Nurgle's side', huh?
This creature isn't of Chaos, but sometimes I definitely have no idea what it wants...