Only real reason to post is if you're going to post something in the role playing board... Which most people don't really do, I would think, but eh.
Alternatively, it is to whine/make a complaint on something in the forums. This is seems to usually be met with hostility in some fashion, or dismissive comments like it will "End Discovery" or so on and so forth.
This isn't a outright rule of thing, of course, but it does seem to be a trend I've been noticing. And if you don't really have much to say in roleplay and you don't really want to join in the debates on things, why post? To tag in with a previous topic someone brought up about registering, if you're not going to pose, why register?