I think the better question is, "Why is fighting allowed?"

It is thought that to get the momentum back in your teams favor, if one of your teammates wins a fight it will get the rest of the team amped up.

Hence why sometimes you hear the commentators suggest that someone should get in a fight to get this team motivated.

Of course this is to be debated, but i would say that often this is a useful tactic, whether i agree with it or not.