Air resistance is friction.
-- Friction steals kinetic energy.
-- A car gets kinetic energy by burning gasoline, and uses the kinetic energy to get where it needs to go.
-- If anything steals the car's kinetic energy, then the car has to burn more gasoline to get where it needs to go.
-- Gasoline costs money.
-- Anything that steals a car's kinetic energy costs you more money to operate the car.
(Another thing that takes kinetic energy away from the car is the car's brakes. The driver uses the brakes when he WANTS to slow down or stop, so it's not stealing, but it still costs kinetic energy and money.)