I think I've done that like twice,... My reason was because for some reason I was suppose to be answering a different question and it linked it to someone else's. And you can't delete an answer so I'd just go it and edit it and say idk sorry or explain what happened. I'm sure this doesn't apply for all cases though.
I've done it because I felt great compassion for that person and their situation. I just wanted to let them know that there are people out there that do care, even if I have no answer for their problem.