Depending on how quickly new threads are being added, a post can go from appearing on the first page of a forum pretty quickly down to being on page 2 or 3. In the Buy and Sell forum, people interested in merchandise are anxious to have it stay in view of as many people as possible----the first page.
By posting any comment, even just the word bump, the thread is moved back up to the top of the cue. By using the word bump, the poster is acknowledging that there is nothing new to say. He or she just seeks to restore the thread to the wider audience.
Yup, it's usefull in the buy&sell, but imho (in my humble opinion) if you have to resorth to bumbing your thread in other forums - your' just not comming to grips with the fact that nobody else is interested. Best kept to a minimum.
Also you usually get PM'ed by the mods if you "bump" threads in other forums (especially in the Pro Forums). Even if you do have new information in your post that is quite pertaining, and it is someone else's thread that you're adding too...just don't make BUMP your first line
But in Buy & Sale it is something that people do, as has been mentioned, to bump their thread back to the first page. Look at that page, wander somewhere for 30 minutes, and go back and see how fast it moves...woah!
I believe it is a dance from the 70's
This question is asked so often as well as a few others perhaps we need a faq section where this stuff could be perminatly posted.