You can't control the car if your wheels are locked up or skidding. Even if your car is equipped with anti-skid brakes (ABS) the ABS normally doesn't work at speeds less than 5 MPH, so when you come up to a stop in a rear-wheel drive car, it is possible that your front brakes will lock up and the front tires skid on the ice while the automatic transmission will continue to drive your rear wheels, and the car will not stop. Shifting into neutral at that point will allow you to control the car better, and allow you to come to a complete stop.