Grocery stores don't have interaction with brands? Are you sure? Why do you think most them have discount cards? It's not because they're being generous.
The discount cards allow them to sell that data to market research companies who analyze which products are often purchased together. They use that data to determine the optimal places to put the products.
You get that discount in exchange for allowing them to track what you buy. The money they make from their interaction with various brands exceeds the discount they offer with those cards, otherwise they wouldn't be offering those discounts.