There are lots of reasons. The posts in a forums are seen in the order of the most recent activity so bumping will bring a post to the top of the forum. Another reason would be that a person doesn't really have anything to add but would still like to show support. Another great reason is that the person has found the post interesting and would like to come back to it. Bumping it will allow that person to track the post using the my replies function in the group tab. Finally, the bump button is still a fairly new addition to the site so some people are just having fun with it.
at 5:18 AM on May. 3, 2010