ETA - in other words, for the gas station it's not about selling snacks, it's about getting customers to their gas station to buy gas instead of the one across the street.
I thought gas stations were all about the extra's these days as the margin on gas was fairly low (~5-6% average, low during price war and possibly negative for some credit card sales). We no longer have the days of the service station with a small office/retail that gave away maps and maybe sold you a pack of gum, with that dirty toilet on the back with an outside door that you would really have to be desperate to use. I have heard that while the gas sales account for over 70% of the revenue, they represent less than 30% of the profits. The margin on the bag of chips, loaf of bread, or quart of milk are very high.