On the consumer side, give a direct discount, and it is too small to matter. Would you really care that, shopping at a grocery store, that you get 10 cents off for each $20 spent (no fractional rewards), when the store charges you 15 cents for each plastic bag?

On the merchant side, many use the information to track customer behavior, and maybe send customized e-coupons. If I buy aspirin, I get emailed coupons for more headache remedies.