rewatchability should indicate a light-weight movie. If you want that, you look at rewatchability, if you want something more complex, you use quality. I think this is the underlying assumption.

Possibly to know whether you should rent it or buy it.

Because the rewatchability rating doesn't just tell you how rewatchable a movie is, but also how good and "timeless" it is, i.e mainly how deeply it resonates with people.

This is not about picking a movie to watch again after you have already seen it once: this is about picking a movie you haven't seen but that is so good that other people like to watch it again and again.