Oh well I'll answer your question anyway. Because 90% of everything is crap (probably more than that on P2P sites). Re-watching a movie can be like walking in the same park more than once, looking at pleasant things with a sense of recognition. You could put a movie in to suit your mood. Sometimes you'd rather watch a good "original" movie again than watch yet another crappy remake or rip-off.
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.
Speaking as a film buff; this is actually quite a good guide to the sort of movie it is (when combined with quality). If lots of people mark it good quality, but wouldn't watch it again, that implies that you have to be in the right mood for it.
If people mark rewatchability high, even if the quality rating varies, you know it is much more easy going film.
And so on. Combining data points is good :)
So rewatchability roughly translates to "emotional connection + quality".
If you've got kids, you know they'll just watch the same flashy animated movies over and over again, even if their opinion of the content is "meh".
What would probably happen is that family films, especially animated ones, would have skewed results.
Plus, they do not normally use online rating systems...