In group posts, the ones that are most recently responded to show up at the top of the list of topics. A topic that hasn't had responses gets pushed further and further down the list and eventually is out of sight. So if someone doesn't have a comment, but they want to help you keep your post at the top of the list so it will be seen, they'll "bump" you. Just by leaving that comment, they help keep your post current.
It doesn't work in the answers section, even though I've seen people do it.
at 1:26 PM on May. 11, 2010