Let's say you post a thread that you want to buy a certain set of chips. 2 weeks goes by without any bites. "Bump"ing your thread is preferable to creating an entirely new one.
When you list your paulsons for sale and they make the second page of the classifieds in 12 hours you too will type "bump".........unless you list them for a buck a chip......then disregard
Unless you just prefer typing dissertations "bump" is a hell of a lot easier.
If you don't want to just write "bump", then do a google search for bump images and pick one you like instead. Images with girls are usually a big hit.