First: as a new non-scientist member of Physics SE I want to say thank you for the wealth of knowledge, interest and patience with my ignorance I have encountered here.
Second: I was very surprised to see that there is a "downvote" function for posts and answers. After trying to think of scenarios where such a function would enhance the quality of scientific discourse I am still at a loss.
The strength of this f̶o̶r̶u̶m̶ question-and-answer website is that it is highly specialised, and it's assumed that the vast majority of questions are not understood or of interest for the vast majority of people, be they physicists or not. I for one would be very busy if I would take the downvote button seriously and start voting on all posts that are "unclear" or "not useful" to me, the same way I upvote those that I find interesting. (Edit after comment: by this I mean that I don't understand 99% of the questions, but that says nothing about their quality, sincerity or usefulness for others, only my lack of knowledge.)
I do understand the 'upvote': it gives an indication on how popular a questions is, which means that there's a higher possibility that I will find it interesting. And it gives me an opportunity to send a small appreciation to those who taken the time to answer my question, or asked questions that enlightened me. But how can the distinction between a post with 0 votes (not popular) and one with -10 (very unpopular) help me to find the information I want?
So my honest question is:
What exactly is the rationale for the ability to downvote?
Can you point me to a specific case here where downvotes has achieved the intended function, that could not be achieved with other tools available? Is there research and/or data that shows that such a button enhances the quality of questions and answers?
Note after comments: I fully understand that the caretakers of this site are rightly proud and protective of it. I see myself as a grateful guest on PSE, and by no means am I criticizing or proposing changes. As a naive empiricist I am honestly curious on how exactly downvoting can have the function stated over and over again on the site and in comments below, as for me it seems counterintuitive. I apologize if this was a provocative issue. Please feel free to express your critique by downvoting this question.