It can allow you to deduce something, depending on the answer. If we want to establish whether scenario A is fair use or not, and we all agree that A is "worse

It can allow you to deduce something, depending on the answer. If we want to establish whether scenario A is fair use or not, and we all agree that A is "worse" (regarding fair use status) than some other scenario B, then if we also agree that B is not fair use, A by definition isn't either. The opposite is not true, of course: B being fair use does not imply that A has to be as well. I find that kind of upper/lower bound logic can be pretty useful and I think it's what the parent comment was trying to do. On a related note, that same logic is why I think Godwin's law can be a bit misapplied now and then. Sometimes bringing up nazis/Hitler can be useful to establish some ground truth in a debate (instead of just a way to imply your opponent is actually a bad person, or, possibly, an actual nazi themselves). E.g. a conversation on the morality of violence is vastly different depending on whether you agree that violence against nazis is ok or not.