This should probably go under the Help Desk park of the forums here.. but whatever I'll answer your question anyway.
People do that because when a new reply is added in a thread, it goes up to the top of whatever little section of the forum where everyone in that part of the forum can see it. So they expect more people to contribute to that thread. It's probably also considered double-posting. And if I'm correct, they're probably not supposed to be doing that.
Bump is a forum specific thing,to add a post to their thread, to let it come up top again of the whole forum thread list. Please do refer to the UAP of the forum, whether it may be done on that forum.