Modern vehicles often use one serpentine belt to drive all accessories, including the water pump, alternator, power steering and A/C compressor.  Due to the high cost of A/C compressor replacement, a vehicle can be rendered undriveable if the compressor seizes and causes the belt to break.  If a vehicle has high mileage, it may not be economical for a customer to replace the compressor when the cost of the repair approaches the value of the vehicle.  An A/C bypass pulley provides an option to keep the vehicle running.