I sometimes posted advice in a comment, because of being pretty sure that the question has been answered before and I just did not find the duplicate yet. As soon as I would find the duplicate, I would add a link.
It would be good to check for a duplicate at first, before quickly firing answer for rep and badges. I guess commenters often avoid adding an answer to a potential duplicate, or to a very localized question with a small mistake to be corrected.
If a question is likely to be closed, adding a comment may be better than adding an answer (which would conflict with the intention of closing).
If it turns out that it should be a real answer, it's a good etiquette that the commentor makes one, or would be asked so. In case there's not response we often noticed community wiki answers made instead.