Comments should only be used to ask for clarifying questions from the OP or to suggest improvements to the question. Comments referring to further/related information are usually fine also.
Comments should not be used to answer questions, give hints, side remarks, etc. This is a Q&A site, so having posts loaded with comments doesn't make the finding answers to questions very clear. We don't want users digging through comments to find answers.
Many users operate under the routine of
IF "Is what I want to say a sufficient answer?"
True: Post an answer
False: Post a comment
But this is not how to view comments. There should actually be two routines going on (that users often conflate):
IF "Is what I want to say asking the OP for clarification or suggesting improvements the OP can make to the post?"
True: Post a comment
False: Do not post a comment
IF "Is what I want to say a sufficient answer?"
True: Post an answer
False: Either work up a sufficient answer and then post an answer, or do not post as an answer or as a comment.