On StackOverflow it is supposed to mean "low-quality post" on reddit it usually means "not interesting" and on the GitHub issue tracker it usually means "I don't think this commit is good enough to be merged". On this forum however it seems to be seldomly used. The only instance I saw of someone using it, resulted in them being suspended: https://discuss.getsol.us/d/1184-disabling-kernel-modules-preventing-them-from-loading/

What do you think is an appropriate use for it?

I never need to use one. I think I'll use it for out off topic replies, or post that is too long but no conclusion.

I don't believe there is any purpose to them, they affect nothing and are purely cosmetic.

It's mostly used to mark uninteresting meta-discussions about the forum.