It's what is called thread bumping and the reason most forums frown upon it is because it can be a way to get a thread back to the top of the "recent posts" board and also increase someone's post count with nothing more than them saying "woo hoo! yeah!" or the like.
Some people start a thread, no one answers and then they just keep posting to bring it back to the top - like an online tantrum. If a thread is really really old, then there is usually a very good reason why, so it's best not to try and resurrect it.
If in doubt, ask a question and someone will direct you to the appropriate thread - and as Sephi said, there are rules, as with any decent forum:
Forum Rules