Retailers know what you bought from them and how much you spend with them. Costco has a better picture than most other retailers because of the membership model

Retailers know what you bought from them and how much you spend with them. Costco has a better picture than most other retailers because of the membership model, but really all retailers engage with this. Knowing, for example, that a customer buys diapers, eggs, and milk once a week may be valuable for an advertiser trying to target specific demographics (like parents).