Free stuff gets you in the door where you’ll be likely to spend money on other stuff.

Building ‘points’ or whatever to get free stuff gives you an incentive to buy things from Kroger you might get elsewhere. Ton of benefits to ‘gamification’ of damn near anything.

And giving free stuff creates a positive experience attributed to Kroger so you’ll be more likely to think positively about Kroger in the future.

Plus all that if they have your email\address they can send you more coupons or whatever to get you back in the door. And a more sophisticated system may track your spending to more effectively influence you through the first three mechanisms. But those are pretty classic ‘customer loyalty’ ideas from before big data was even a thing.