I know it's sounds strange but Michelin started rating restaurants for it's road guide, aka paper maps, when we didn't had gps and phones. It did made sense to point out the best places to eat, and it ended up becoming another industry out of nowhere
Likewise, who would remember to rotate their tires without a handy time-keeping book? And who would remember to look at it every month without an incentive? Clearly, the supermodel calendar is a very tire-centric design.