From what I know, some people say it is due to the pattern of the shirt or to ensure that the button stays buttoned, but there is no real reason for it.
I'm ashamed to admit it, but I think that I know this one.
It's so that when your shirt is tucked in and you sit down the shirt tails will be able to slide just a little bit to the sides. If that's not why then at least it sounds good.