They can prevent you from sitting in some pretty nasty shit....
I've never worn one, but sometimes I wish I do cause I find myself on the ground a lot.
Other than that... eh. I dont mind kids who wear them for the most part. But some are rediculous. haha
Really! I think that would be an intended purpose. Also, it covers any holes you have in the ass region of your pants, and is a flag for whatever band or movement you really follow.
It seems like they'd come in handy, because my pants basically always hole first in the ass-pocket region (what, is my ass constantly expanding?) but they usually just make people look like they couldn't decide between pants and a kilt that day.
If they are like the ones on google images (a big rounded leather flap tied round your waist) IMHO, I don't like them! heh heh But that's just my personal opinion. Maybe if they were like square cloth sections, like for patches I would though.
bum flaps... as gay as it sounds... are both a extra part of clothing, protection from people seeing your holes in you pants, and an ommage to the early american indian. I personally wear one just because i like it.
for those who dont know what a bum flap is, heres an example: