The twenty digit code packaged with the first shipment of Final Fantasy XIII gives future Final Fantasy XIV players a secret item when the MMO debuts next year. Before that happens, the campaign code is also a ticket to apply for the PlayStation 3 Final Fantasy XIV beta.
Square Enix did not announce how many Final Fantasy XIV beta testers they will accept, but if you want a chance at the PS3 Final Fantasy XIV beta in Japan you need a campaign code. As of now, there’s no other way to get access to it. Anyone, can apply for the PC beta, though.