It’s a way to keep people shopping at the same store. Since most fuel perks are something like 10 cents off a gallon for every $100 spent, if you spend a lot of money at a store then you get more off of a gallon. It then discourages you from going to a different store because you may not have as many fuel perks and would have to spend more to get the same fuel perks you may already have with the first store. The stores of course don’t want you to go to their competitors, so it’s a win for them.