What was the point of the time turner in Harry potter ?

Okay, so in the 3rd Harry potter movie, Harry and hermione go back in time to try and save sirius and buckbeak, but I don't see the point of it. Because, everything that they did while they were going back in time had already happened... Like, they didn't change anything. Buckbeak never died because in the present, the slayer just chopped a pumpkin, not buckbeak, but they just couldn't see it cause they were standing behind a tree. I'm so confused.