There isn't really competing theories. The reason why its done is very well known, and others have stated it: fighting provides immediate consequences to players that break the moral code of the game.

Whether its a good thing or not is debatable, for sure though. I'd personally argue that if there was a better policy to achieve the same results, we would use it, and fighting actually has gone down as a result of high definition cameras recording more of the games. Players get away with less bad actions and there are less fights.