What does anti-lock braking system prevent when used in an emergency stopping situation?

Answer 1

(original answer ):

"it (ABS) prevents your wheels from locking. This prevents skiding or spinning out and allows more control when trying to turn and avoid danger"

(expanded): A wheel that is not turning cannot practically control the steering. It is basically an inert block of rubber sliding along the road. The front wheels must keep turning to steer and the rear wheels must keep turning to follow this direction and maintain stability. It has also been demonstrated that a wheel that maintains some minimal revolution will stop in shorter distance than a locked one. Before ABS, racing drivers used "cadence braking" techniques to quickly release and reapply the brakes continually to achieve best control and stopping power before wheel lockup occurs. This is what ABS does automatically and at a higher rate than humanly possible and with greter accuracy.

All drivers should try their ABS at the limit in a safe location and note the unusual feel of the pedal as it vibrates underfoot - and learn to trust it.

In an emergency stop, many drivers have panicked at this unusual sensation/vibration and have released the brake with disastrous effect!