Rumor also has it that in virtually any major grocery store in the USA you can put in any (valid) area code and 867-5309 and it will work as a rewards member number to get you the discounts. I have done it when traveling and have had success.
I would assume that as many are using it for the discount, there are at least a substantial portion who use it for gas discounts at Kroger gas stations. You can get up to 30 cents per gallon off if enough has been spent lately.