Poseidon is Kronos' most favourite child and always has been. His sweet youngest boy (he's ignoring that Zeus exists because he's emotionally disowned him), who looks exactly like him except for his beautiful eyes which are all Rhea. The only reason Kronos even ate his kids was to protect them from his own father who was still a threat at the time. And by the time the matter was resolved, Zeus freed his siblings and turned Kronos' precious babies against him in battle, banishing him to Tarturus.
Kronos is now trying to come back and take over again. Not because he desperately wants the power but because he is disappointed in how Zeus is ruling. Kronos has always been family oriented and Zeus ignores or mistreats his children and siblings. Not to mention the way Zeus' paranoia keeps Poseidon away from his brother Hades in his absence. Even though Poseidon is now seen as one of the oldest gods and has many children of his own and is a king in his own right, he's still Kronos' baby.
Kronos had used Luke to return, using his body as a vessel. Kronos has never intended to destroy Olympus or end the reign of the gods, He had simply wanted to teach Zeus a lesson. he knew Luke wanted to destroy Olympus but he believed that he could overpower Luke after manipulating him to help Kronos return. But he made a mistake. He didn't anticipate how much control Luke would have over their body. Luke gave orders that he pretended were from Kronos, orders that horrified the Titan, orders to attack kids (the young demigods), orders to release monsters and plan attacks and worst of all, orders to Oceanus to wage a war against Atlantis (his treasured son's kingdom and home). Basically Luke was behind every truly horrible thing that happened, not Kronos. And Kronos kept trying to regain primary control and stop Luke but he was still weak from the ages spent in Tarturus.
It is now the Battle of Manhattan, with his grandson, Percy, leading the demigods into battle. Kronos wished he could stop this. They were children, precious lives, and his favourite son's dear child. He didn't want to hurt them.
But what allows Kronos to finally gain control and stop the war is when a dangerous direct attack happens on Poseidon as he fights the battle and aids in the fight against Typhon. No one harms his baby. Kronos protects his treasured child and eventually is able to end the war by switching sides to fight against the army Luke had created to destroy his children.
He knows he wont be allowed or trusted on Olympus so he literally just explains in the wreckage of the battlefield exactly what had been going on. He asks for help so he can have a new vessel that isn't Luke and they can kill Luke so none of them are harmed ever again. Zeus is paranoid but Hestia remembers the kind side of her father and as the oldest is able to help him recover his own physical body so they can end Luke, the threat to their family.
Other than Hestia, Poseidon is the only one who remembers their father's softer side. Kronos had taken a long time to actually eat Poseidon to keep him safe with his siblings as he was unable to force himself to part with his precious child, the one he cared most about. He had actually raised Poseidon for a good period of time before the god joined his siblings.
Now that Kronos apparently isn’t evil, the demigods get to see the blatant favouritism towards Percy's dad, who Kronos obviously adores. And Kronos easily still considers Poseidon his youngest, clearly having disowned Zeus. Other than Poseidon, his second favourite is Hestia, his two most loving and family oriented kids.
Percy gets to see his dad get treated as if he's a kid again considering how fussy and clingy Kronos is, just trying to make sure his kid is okay and wasn't harmed by the attack.
Zeus gets the scolding of a lifetime for how divided the Olympian council, their family, has become all because of his paranoia and rules. Kronos revoked Dionysus' restriction from drinking wine since its his main domain and being cut off harms him. Kronos is also happy to meet his godly grandkids and to see his children again. He also gets rid of Zeus' unofficial bullshit about Hades and Poseidon keeping their distance so that his pearl can have his big brother back (he knows they had become close after all the time they spent together in his stomach).
