In most cars is power from the engine is used to drive either the front wheels or the back wheels. This contact with the road provides sufficient grip on all normal road surface. But when the roads surface is slippery such as when covered by snow or ice, two wheels may not get enough grip and one or both may spin. With all four wheels being driven the whole weight of the car is being used for traction and a better grip is obtained all round.
Military vehicles and those designed to travel over rough country are usually equipped with four wheel drive. For running on normal road, these vehicles are driven by two wheels as four wheel drive is unsuited to prolonged higher speeds. An extra gear lever is used to engage four wheel drive which usually operates at a low gear ratio.