From a moderation perspective question downvotes do at least one thing: they feed in to the automated site question and answer ban systems and can, temporarily at least, limit the flow of qualitatively bad questions from a user. 

Sure there is nothing stopping the user from creating a new account and starting again, but it provides a clear signal that we have a minimum set of expectations from users.

From the page detailing [why a question ban][1] might be applied:

> At Stack Exchange, asking questions is a privilege, not a right. If a user cannot demonstrate, after asking a reasonable number of questions, that their contributions are making the site better (or at very least, not making it worse) then they will be blocked from asking further questions until they have demonstrated their ability to contribute positively.

Sure, in the words of Mr. Garisson, "there are no stupid questions, only stupid people" and everyone deserves the chance to ask a question, *some* people insist on asking questions that are off-topic, are lacking every key detail needed to provide an answer, or are just not very good.

Downvoting questions tell not only other users but the very system itself the "this user is not asking good questions" *for whatever reason*. 

I would defend everyones right to ask questions, regardless of how "stupid" they might be, but if a user is asking questions in "bad faith" or are otherwise abusing the system in ways that fall under the radar of site moderators then downvotes give normal users a means to effectively moderate the community without resorting to calling a grown up in the form of a moderator or contacting the community moderation team.

Downvotes fit the style of *community* moderation. They point out *bad content* and have consequences and are not simply about giving OP a smack. 

Being told [that what they are doing is not acceptable as well as what we expect from them and what they can do about it][1] is a useful feature that completes the ***community*** moderation style of the Stack Exchange sites.

Hopefully someone would have spoken to them before it got to that stage, but sometimes the only way to make people listen is to force them to stop first.


  [1]: https://superuser.com/help/question-bans