I've noticed more and more super/hypermarkets started asking for your zip/postal code sometime during self-checkout. I'm guessing they use these as approximations about where people travel from, so they can evaluate if to open more stores closer to popular areas, or something like that. Pretty sure there is more use cases for postal codes too.
>>diggan+(OP)
Postcodes are very useful (but not perfect) proxies for household socioeconomic status, which is useful for marketing and sales analysis.
That data linked with the payment method that the register collects pretty much gives the store exactly who you are and where you live even if you chose not to sign up to the store's loyalty program.