i have a forum and i let everyone to do everything except posting (when they are not registered) but i have seen that many people joins the forum but don't post anything, so whats the use of registering?

they want to feel special joining a forum xD, but some people just join for the heck of it or they join then later find it boring or they don't like it anymore and leave.

They are allowed to. This is the reason. *Facepalm*

That's a great question.

I think that most people find a forum interesting, they register, thinking that they'll come back tomorrow and usually forget.

At least that's my opinion

The only way to make someone register and post on your forum is to own a "pay2post" forum where you pay them for each post they make.
But that isn't really a good idea.

They register, and then they don't feel the need to post.