Let's say I commented on this post:

This is totally wrong, I don't like your thinking and your attitude will only make Steem price go to 0.

If you have anonymous downvote and you are butthurt, you would downvote me into oblivion. Then I would not know who did it, but I would probably think that's you, even tho' this is not the only post I commented on.
Then you can say you didn't downvote me which will make me question other authors of the post I commented on...

You know how it goes...