Arguably the most important part of any narrative is character motivation. Giving characters a reason for their actions and habits is an integral part of constructing a three dimensional character. Not only that, but knowing a character's motivation also gives motivation for the player. Of course, the motivation for anyone playing a game is to, well, play the game, but adding that motivation in can give the player something extra. It can help them really understand the character they play as and the other characters they interact with. It helps to better immerse them into the game's world and enjoy the game that much more.