As others have pointed out, the backwards brim is 100% to be able to suck and not lose the hat. Unless you're so low that you're basically looking straight up, baseball hats and bjs don't go together.
As for the why, it can add character. It either shows that they're a jock (a gay fetish), or adds a bit of their personality to the video. If they're a porn star, it can also help that scene stand out from others they've filmed like it.