"Good"/useful submissions will (hopefully) get upvoted anyways, so there isn't really a need to downvote the "bad" ones. It could also be considered a deterrent

"Good"/useful submissions will (hopefully) get upvoted anyways, so there isn't really a need to downvote the "bad" ones. It could also be considered a deterrent for new users who are still learning what is "good" to submit, or it could be used for an automatic downvote bot or something.