It's to stop spambots from auto-reg'ing/link dumping.
That's not the only reason we have this system. Part of it is that we want you to read and research stuff on your own before you post in the main forum. On other forums, if you were to not read anything and a post a thread that has been discussed thousands of times, they would lock the thread and ban you. We don't do that here, because we believe in personal responsibly and free-speech. Because of this interesting dynamic between the light-moderation and the difficulty of making a new account that can post, this forum mostly moderates itself. In the cases where it doesn't, people will at least regret getting their account banned, because they have to put a few hours into a new account to bypass the ban (note: that is a bannable offense*, but I'll admit that we don't catch everyone who does it).
*It's only bannable if you bypass a ban. You are otherwise allowed as many accounts as you want as long as you don't scam people or get banned.