Sake means the purpose for doing something. You might run a fund-raising marathon for the sake of sick children.
Sake comes from the old English term for "affair," or "cause of guilt." We usually use sake to talk about the motivating cause of something we do. You might study hard for the sake of your grades. But then again, you might blow off your studies for the sake of having fun. Spelled the same way, but pronounced SAH-key, sake is also the name of a Japanese alcoholic drink made from fermented rice.