If the car's gear slips (because the gravity has pulled so hard on the car that it actually made it slip out of gear), it wil roll down the hill. If the wheels are turned to the left while parked uphill, then the back of the front tire will bump into the curb and this will prevent the car from going anywhere else. When parked downhill, the wheels turned to the right (I am assuming that we drive on the right side of the road) will make the car roll until the tire hits the curb, thus preventing it from rolling downhill.
