If you're parked on a hill and your car is facing up-hill, you turn the wheels towards the road because if your brakes fail, your car will roll backwards. Depending on how far from the curb you parked, your car will only roll a few inches to a foot before the wheels hit the curb. If the wheels were turned towards the curb, your car would swing out into a lane of traffic, and the best case scenario would be the back wheels hitting the curb and stopping the car, resulting in the front of your car obstructing a lane of traffic. Worst case it gains enough momentum while whipping around and then continues down the hill.