If it's a stupid reply and doesn't answer the question. If it's from one of those grey silhouette profiles.
If it's a hubber who I don't think is serious. If it's someone who I think is just trying to get merit for giving an answer, just any answer. I've had more than a few who have answered with "hello I don't know the answer but I'm glad you asked the question".
This drives me demented and I generally delete them. In fact of late I don't think I have bothered with the vote down, I just delete nonsense,