Sake is a word that is almost always used in longer phrases like “for the sake of,” meaning “for the benefit of.”

When your teacher asks everyone to be quiet for the sake of the students trying to concentrate, she means “to help those students.” When someone does something for its own sake, they do it simply because they enjoy it, not to get anything else out of it.

You'll also hear people say “for goodness' sake” or “for heaven's sake” when they're frustrated or want to emphasize something: “For goodness' sake, please remember to close the door!”