The more you buy, the higher your level will be. It's meant to be "a neat way to show off how big of a Square Enix fan you are." The program seems modeled after many frequent flier programs, with four tiers of membership fans can receive: Bronze, Silver, Gold and Ultimate. Tier levels reset at the end of March each year, so you'll have to spend, spend, spend on an annual basis to keep up your membership.
Don't bother dusting off your anthology of Final Fantasy VII games, though. Product registrations will begin with "every new Square Enix game" starting with VALKYRIE PROFILE: Covenant of the Plume, available in stores today.