My guess is advertising.

Locally, we have two regional convenience stores that offer $0.03 per gallon discount if you use their Rewards card.  A day or so ago, I was going thru my email, and I noticed one of these emails from one of the convenience stores. I decided I am tired of getting these emails (advertising emails), so I decided to unsubscribe.  When I did that, I got a message that said, "If you unsubscribe, you will lose your $0.03/ gallon gas discount. Do you wish to continue?"  

So, my guess is there is value in the advertising, and that is probably where your nickel discount comes from.