What 's the use of the "comment usefulness" bar?

You will notice that in the comment box, there is a horizontal bar entitled "comment usefulness". This bar shows you how useful the comment has been to other readers. If no one has rated the comment yet, instead of a bar, you will see the the words "Usefulness:unknown", as in the picture below.

You can rate the usefulness of another person's comment by using the "Rate Comment" dropdown list. As long as 1 person has rated the comment, the bar will be colored and show you the average "usefulness" level of that comment.