There's a few reasons. First one is for the vocalist. It makes reaching the high notes a little easier. Second is if you have a 12 string guitar in the band, most manufacturers suggest tuning em a half step lower cause of the extra tension. Third, is cause it's easier to bend notes, so if you do a lot of lead playing, it's easier on the fingers.
Other reasons are just to do something a little different....most bands tune to A440 which makes all bands sound the same, go one step lower and it frustrates the heck out of beginner guitar players