This is a really good question!

1. As others have said, the idea is to study something before you forget.

2. It's hard to predict when you're going to forget something, so you do wind up studying a bunch of stuff before you really have to. It's a limitation of prediction (and also of the technology as developed so far).

3. It really is pleasant to work to recall things even when you succeed at it. It does "freshen them up" in your memory. And sometimes just the experience of seeing a fact can be pleasant. (A lot of us review familiar things for the joy of it in other domains--movies, etc.)