I _think_ this rule is intended to discourage "Have an upvote" (and similar) comments, commonly seen on Reddit.
Reminding fellow readers/commenters to not just shallowly dismiss comments once in a while doesn't do much harm, and actually lead me to reply to the parent.
"Please don't comment about the voting on comments. It never does any good, and it makes boring reading." - https://news.ycombinator.com/newsguidelines.html