The primary effect of upvoting a submission is to make it sort higher relative to its present position; the main effect of a downvote is to make it sort lower.

The primary effect of upvoting a submission is to make it sort higher relative to its present position; the main effect of a downvote is to make it sort lower. So downvotes are a mechanism to add information about your preferred (partial) ordering of comments in a thread. Upvotes alone only get you halfway there.