Radiant Garden is where it all began, and, as such, it features heavily in Kingdom Hearts' lore.
Before the first game’s events, Maleficent and her horde of minions overtook the world, transforming it into the bleak and forbidding Hollow Bastion that players visit during one of Sora’s lowest points.
It’s the exact opposite of Radiant Garden: home to terrors, darkness, and loneliness. But after the massive battle in KH2, efforts are underway to restore it to its former glory.
That’s fine and dandy, but it isn’t until Birth by Sleep that you realize why Radiant Garden was so important to begin with. It was basically the home of everything good in the game — Kairi, Mickey, light in general, and the meeting ground for the forces of good.
But as goodness is want to do, it gave birth to the darkness as well.
Radiant Garden is essentially the KH universe’s barometer — thriving during the time of light, barren after darkness overtakes it, and striving for something better during Sora’s trials.
What happens to Radiant Garden will largely reflect what happens in the story and characters, whether it’ll still be the symbolic center of the universe, or if it even should be, is another matter entirely.