There is a post on How do comment voting and flagging work?, but I didn't find any post on the philosophy behind it. I see people often mean "me too", "I agree" or "I like it because it's fun" by a comment upvote; "this comment really helps" seems to be quite a rare message.
Are all these reasons to upvote a comment considered "good" or at least "OK"? Has there been any discussion or is there a consensus about this? And finally, as an optional subquestion: what were the supposed reasons to upvote when comment voting was implemented?