Supposedly, aesthetics.
That's it, though. Big wheels are heavier, and it's some of the worst weight you can add to a vehicle (rotating *and* unsprung). They have negative affects on braking, acceleration, AND mileage, and any benefit for handling by making the sidewall smaller is lost from the costs of added unsprung weight.
Basically they prove that most people are totally fucking ignorant when it comes to cars.