Dear imgur. why is there a downvote button?

I can only think of one reason for a downvote button.. and that is to show disapproval of a comment.. But what happened to if you don't have anything nice to say don't say anything at all? We learned the phrase as children but it applies to adult life as well.

All I'm saying is maybe we don't need a downvote button. After all, the true measure of a comment's strength should be not so much how how many people downvote the comment to make their own look better relatively, but how many people like the comment enough to give it an upvote.