There is a 7 day secret, private beta so that a few hundred super-committed people can fill up the site with questions to set the tone, and establish a system of tags. That way when the public beta hits, the site is already stocked with a bunch of appropriate questions.
After the private beta, there is a 60-90 day public beta. The purpose of this is to make sure the site really survives and doesn't just get a burst of activity and then disappears. Sites which still have a lot of activity at the end of 90 days become permanent sites... sites which don't would be closed down.
The public beta also allows a community to start to develop.... in addition to setting a tone for the site and deciding what kinds of things are on-topic, they work on picking a permanent domain name, a logo, a graphic design, and electing moderators.