Well, any post you make earns you gold, but over a twelve hour period, the more you post, the less you earn per post. Every twelve hours it resets, though. So yes, bumping will earn you some gold, but really it's not worth bothering with.
The other reason to bump is to keep your topic in a position where others will be likely to notice it, read it, and reply. This is only allowed in some of the more fast-paced forums like the Exchange, and of course, the Chatterbox.