Downvotes help clear out false but emotive posts.

The sort of "gangs of [minority] on murder spree in [town]" posts that get so much traction on Facebook.

Even if most people dismiss it out-of-hand, it's so inflammatory that the gullible are much more likely to interact and you get exponential spread.

Maybe HN is just better than that, but I see it as a major differentiator of Reddit vs Facebook.