[quesera]

ZIP codes are used as a weak "something you know" factor in payment card processing.

The card is (for card-present transactions) "something you have". And the ZIP complements that. ZIP code is optional, but the merchant gets a data integrity score back from the network ("AVS/address verification service response", from no match to full match), and can accept/decline the txn at their discretion.

Because it's optional and at merchant discretion, all it really does is give the merchant some additional ammunition when disputing a chargeback. And of course to build a demographic database.