A common criticism of the Kingdom Hearts games is that the Disney worlds have no bearing on the overall plot, being nothing more than fillers.
I'm here to argue the contrary, that not only they are incredibly thematically relevant to the franchise, but that they represent its entire ethos.
Kingdom Hearts is a game series all about how even in a dark universe full of tragedy and sorrow, there are still moments of whimsy and playfulness, and not only these moments need to be protected but they are what make dealing with the horrors of life more bearable.
In almost every game there's an apocalyptic threat that will destroy everything and will drag everyone to the darkness, and the protagonists need to travel to the Disney worlds in order to protect them.
And the people from these Disney worlds are hopelessly clueless about it, having no idea that at any time their worlds and their entire livelihoods can be completely destroyed. But they are innocent, sincere, and playful, and by protecting them they become your friends.
You get to spend time with them, to have fun with them, even with overall world falling to pieces, even with all the tragedy and pain still going on elsewhere.
And when the climax comes, when it's time to face the bad guys, it's the moments you spend with them, it's what you learned about friendship with them, that makes you able to face Xehanort and his many incarnations. It's by spending time in the light that you find forces to face darkness.
You have multiple cases of this through the franchise.
In the first game, it's Sora being distressed that Riku aligned himself with the Heartless and Kairi apparently lost her heart, but the idea of telling her that he flew with Peter Pan is what brings him joy.
In Birth by Sleep it's Terra that has been struggling with controlling his darkness during the whole game, facing Disney villains in almost every other world, but then he has to protect Cinderella on her way to the ball and it's her faith in her dreams that inspires him to continue fighting.
And you have the most obvious examples, the Winnie the Pooh worlds where Sora assumes the role of Christopher Robin. There are no heartless in this world. No violence. Just you getting to spend time and protect the literal embodiments of childhood innocence.
That's the core of the whole franchise, it's through these silly childish minigames, to spend time and have fun with these silly cartoon and fairy tale characters, that you gather strength to face the darkness
I feel like Square Enix gets Disney better than Disney themselves. They get the appeal of Disney movies, those nostalgic stories that stay with you forever, even when things are dark. I do risk saying that in several instances Kingdom Hearts has the DEFINITIVE versions of these characters and stories.
When Disney writes crossovers between their properties they tend to get hyper saccharine, with a heavy focus on the magic and the wonder. But Square makes the Disney characters matter thematically in the games. It's only when the sweet and the whimsical is juxtaposed with the dark and the tragic that we are able to appreciate it better.
They don't take the "Disney Magic" for granted. They use it to tell a very convoluted but deeply moving story about how making friends and hanging out with your friends is what truly matters.
In the Kingdom Hearts games Disney are not the storytellers, they are the medium used to tell this story of pain and loss, but of discovery and wonder.