Why
When the brake pedal is depressed, the wheels can lock before the vehicle comes to a stop (skid). In such an event, the manoeuvrability of the vehicle is reduced or totally lost if the front wheels are locked (The vehicle will travel in the direction its mass weight takes it irrespective of the position of the steering wheel). Likewise the directional stability of the vehicle is compromised if the rear wheels are locked (The unrestrained mass weight of the rear of the vehicle will try to cause rotational movement, i.e.- a spin).