There are a few justifiable reasons to wear a cap with a directional brim backwards (I'd say baseball caps, but most of the ones I've worn are actually patrol caps... fuck baseball, our national pastime is actually war).
1) I'm traveling up a hill, and I need to look up further than the brim allows. I'll turn it around while I climb the hill, then put it back to block wind/rain/sun once I'm on flat ground again.
2) Underneath a helmet to keep rain from going down the back of my shirt while laying in the mud with a rifle. Seriously, it works.
...and that's about it. To anyone who wears a brimmed cap for fashion: fuck fashion. Get your priorities straight, poser.