Not all tires rotate at the same speed, especially when you are turning your vehicle.  A differential is critical in making sure the engine power is regulated to your wheels.

Car wheels spin at different speeds, especially when turning. Whether you drive a rear-, front-, or all-wheel-drive vehicle the wheels spin by way of a differential. The differential regulates the engine power to the wheels, allowing each to spin at a different speed. The gears inside the differential slide around coated in a slippery film of oil. Just like engine oil, the differential oil must be changed at regular intervals.