Why do people replay single players games they've already played before?

I'm talking about those that play the same games 2 or more times. You already know how the game plays and you know how the story goes. Wouldn't you rather spend that time playing a new game? I've only got so much time to play and a I have huge backlog. I never found myself picking an game I've played already over a new one. I'd like to know your thoughts.