What happens in the wind waker when you go in that cave in the back of dragon roost island?

Answer 1

Something very important. You get to the Wind Shrine, which is only accessible this way. Here you learn the Wind's Requiem - a song on the Wind Waker that allows you to change the direction of the wind. The game is almost impossible without it: you won't be able to sail to an island if the wind isn't going in the right direction. You won't die or anything, so don't sound so scared!

Answer 2

Inside the forest on Outset Island, is the fairy spring that will increase you wallet size. (Bombs required)