They have to have a way of distributing keys and whatnot.

Also, signups may be necessary to get a rough handle on how many servers they're gonna try to rush and put up. I honestly doubt they have any more than the Closed Servers and the Stress Test servers ready to be brought online at the moment.

The new Stress Test will give a rough idea of how the servers will perform under a heavy load with the new game version (since the last stress test), and the Open Beta signups will tell them how many servers they'll likely need to accomodate the OB load when they're ready to begin.