Well, aside from the thrill, adrenaline, and endorphin from kinky spanking, it's just a great turn-on to those of us who love adventurous sex! I think that it's the hottest, most intimate and intense kind of foreplay. I've seen many people say that they get release and great satisfaction changing their daily dominant or submissive role to the opposite in their private sex life with the one they love and trust. See this great article that should hit home; ask what she thinks about it: http://www.thefrisky.com/2009-06-01/first-time-for-everything-spanking/
I cant definitely attest to rough sex as a whole, but im particularly into debasement and verbal abuse because its the total opposite of my nonsexual way of conducting myself. Someone not in a dom position, at an appropriate moment calls me a gutter whore and we gon fight coz im the only person allowed to make me legitimately feel like shit.
This is actually pretty common. lol. I don't think it's something you can really put a reason behind, in most cases. You like what you like at the end of the day. You can't really explain why your favorite color is what it is or why you like a certain flavor more than another. It's just what you like. As for physical roughness, it could be because the vaginal cavity is much less sensitive than the clitoris (and also less sensitive than the penis). Personally, I feel that the rougher the sex is and the more sensations are involved (spanking, kissing, biting, etc.) the more physically pleasurable it is. My boyfriend and I don't have many mutual kinks, but we do spanking, biting, that sort of light stuff. It really does make it feel better in a physical way. In some cases it may make the female momentarily "tighten up," too, which can lead to an increase in physical pleasure. But for the emotional side, I think it's pretty inherent. People just like different things. Some preferences (sexuality, colors, racial preference, food, et al.) just defy explanation, I suppose!