As mentioned before, community subtitles are for crowdsourcing subtitles. The main benefit to this that I can think of is the potential to outsource *translations* of subtitles, so that one doesn't have to rely on poor automatic google translations or pay a professional translator. Or, it can even be helpful to outsource the actual subtitles for those who do not have the time to provide their own (e.g., they didn't have script prewritten and find it to be too time consuming to write out the script after the fact.)
I think it's actually a pretty interesting idea, if you have fans who are dedicated enough to contribute. I'd love for some of my covers to get translations in Japanese since they are so often covers of Japanese game music, but alas...no one has ever used the community contributions for any of my videos.