So long as people's value functions have a directional bias in hyperdimensional value-space, the averaged value function embodied by the upvote-downvote tendencies is useful.

(If there were no bias- if people's opinions/tendenfies varied equally likely between all possibilities- all comment scores would be 0. Of course, practically survivor bias soon sets in, as radical suicide advocates wouldn't stick around long enough to like/dislike much.)