I first notice the mail in Gen III but never really payed much attention to it, but now im just wondering what it is for?
You write messages on it. And when you trade, the receiver gets to see it. Not a big purpose, really.
Mail's always been sort of pointless, since you can't really spell much out, but its fun to send it to friends through trade. Its for simple things, like saying 'best' or stating the pokemon it is. Like Mew said, it doesn't have a actual purpose.