Karma as a feedback mechanism allows users' perceptions of whether someone is contributing positively or negatively to take on lasting status associations, in a way that comments do not. Plus, it encourages users to provide feedback more frequently than they would if they had no way of doing so other than leaving comments. Karma would most likely continue to fulfill these purposes even if people leaving downvotes or upvotes always explained themselves on request, as requests are infrequent enough that the possibility of having to explain oneself would probably not be a powerful deterrent to voting.