Not fighting can lead to a drawn out conflict that never fully ceases and develops into a feud.
The key to the whole thing is that both parties engage willingly in the fight, and neither seeks to do any serious damage.
Think of it like a "gentlemanly" duel, not a fight.
It also teaches that controlled aggression is better than uncontrolled aggression. In other words: fighting with rules > fighting with no rules.
To win in a controlled fight, you must be capable of some degree of discipline. If I am fighting someone, anger is my worst enemy.