A big factor in many video games is a sense of immersion- the feeling that you yourself are involved in events playing out in another world. Many games try to do this by implying that the protagonist is simply an extension of the player- an avatar. The trouble with avatars is that giving them the potential to do anything makes it hard to make them do... well, anything. Thus other games prefer to give you characters with a life of their own, you just get to play as them. So, here is the Sliding Scale of Player and Protagonist Integration.