A reason for XP is to serve as some kind of yardstick to your progress. A non-linear quest (such as in the Fallout games) uses XP to give you new abilities the

A reason for XP is to serve as some kind of yardstick to your progress. A non-linear quest (such as in the Fallout games) uses XP to give you new abilities the more you play the game, whatever you happen to do in it. Whereas in linear stories you can simply present the hero with whatever bonus their progress deserves. Note that you can have the character-improvement mechanic in more than RPGs; a game like Half-Life introduces weapons, equipment, and new ways to manipulate the environment as you progress.