I think you're asking about your own comments, rather than those of your visitors. You're given this option in case there's any reason you need to hide your own comment on your hub.
This happened to me once. We're not given the option to delete our comments, and I had made a pretty embarrassing mistake on a comment on my own hub. Fortunately, I was able to hide it by clicking the "moderate" button, and I then re-wrote the comment.