From December 5, 2012
From Episode 309 of the Anime
Urahara: When you say that thing? Are You Talking about the Soul King? I See. So You Saw It Didn't You? Without the Existence of the Soul King, the Soul Society would split into pieces. The Soul King is the keystone to it all. Without that Keystone in place, the system would fall apart. That my friend, is how the world works.