This is a not so commonly employed courtesy where, if one person provides transportation the other(s) being transported will pay for the activity they are going to.

How about an example:
Lets say you and your 3 friends are going to catch a movie at the local theater. You don't want to take 4 cars to the show, so instead you carpool to the movies. It would be a kind gesture for the 3 passengers to pay for the driver's ticket to the show. Why? The driver put up with traffic, fuel costs, and his cargo. You most likely didn't want to drive there yourself so you could use this to persuade a friend to drive while you just chill in the passenger seat.

If you are stuck with driving and you could suggest the inverse: "I fly you buy". This way you get a free ticket, meal, or what not for doing something you aren't to thrilled about. Besides, what is an extra buck from each person for transportation. Sure as hell beats a taxi. In my mind this makes sense. I always buy when I get a free ride, and it always is well recieved by the driver.