the author may upvote your comment as a thanks or acknowledgement of having seen and read it if a reply isn't necessary.
I always go through comments and upvote them manually. May be @commentrewarder can be a good tool for them who don't. But then the question is the actual reward that a comment gets, it's not that big, right ? So it would be helpful for new users more likely.
It helps those who can't vote things past the minimum threshold.
Ya.