In addition to allowing popular comments to be promoted I believe comment voting can also be used as a form of phatic expression.
In linguistics, a phatic expression is one whose only function is to perform a social task, as opposed to conveying information. The term was coined by anthropologist Bronisław Malinowski in the early 1900s.
For example, "you're welcome" is not intended to convey the message that the hearer is welcome; it is a phatic response to being thanked, which in turn is a phatic whose function is to be polite in response to a gift.
or to put more succinctly to be phatic is to be "full of social gestures that are like apes grooming each other"1
I've often upvoted comments on my own answers that were thanking me for an answer as a feedback to the OP that I have read it and appreciate the thanks. At least I hope that's how it would be interpreted as I was the only person that would be interested in the comment.