In minecraft why would I want to reset the nether?

Does it do anything to the enemie population or make a nether fortress appear if there's not already one

The reason why nether-regeneration is there's (I assume that this is for Xbox 360) so that if the world was spawned in an earlier update, then there would be no nether fortress. Another reason is if there's no blaze spawner (which can happen on rare occasions), then you don't have to delete the world completely.