I have only used down votes for inappropriate questions. I don't know if that's the purpose - or perhaps there are other purposes that I have not thought of.
There was one that was extremely angry and rude by a brand new member who demanded exclusive "ownership" of "her" profiles created from "her" gedcom. I used a down vote to indicate that I thought this member could stand some attitude adjustment. It's one thing to seek information about how WikiTree works, but it's another to criticize it and demand that it be changed to suit her pleasure, while being rude to the people trying to explain it to her.
There was another that was essentially an advertisement. A person with a business of finding missing heirs to collect their inheritances was seeking volunteers to help find these people. I flagged that, in addition to giving it a down vote, and it was promplty removed - apparently whoever gets notified of flags agreed that it did not belong here.
Frank, I agree that this is a VERY good question - there should probably be something someplace here that discusses the when and why of down voting.
Thank you so much for asking, Foster!