Many members also get the credit card/member card. Which means they've also done a credit check. Memberships probably also have age and gender info which is use

Many members also get the credit card/member card. Which means they've also done a credit check. Memberships probably also have age and gender info which is useful for tracking against purchases. The membership model also means it would be really easy for them to add certain allowances for data collection (eg, in-store surveillance tracking to see where certain age groups go in the store and where they linger and what they look at.)