To prevent characters from gaining an unfair advantage over other characters and become more dominate. In other words to prevent someone from being the defacto 'strongest dude'. My guy is build as a 'melee tank' sort of dude for example. The trade off being he is garbage at range. Someone else can cover that. In my opinion it is not very fun to be in a roleplay where I can say 'Oh just send it ____ because (s)he can just take out the problem like that'. It creates a better experience for everyone so that they can work together to cover each others faults. Besides in the case someone does end up in a PVP situation for one reason or another in roleplay by their own wish or story it prevents one character from just steam rolling the other...unless their opponent is within their weak point in which case can't do much about that. 

My second reason is to prevent the players from doing this too serious NPC enemies. What's the point in fighting a 'boss' character when the character just whams the guy in the face and the 'encounter' is over. I prefer fairly even to intense fights.