Concave mirrors are used in headlights of car. The position of the is placed nearer to the focus of the concave. The reason is that the main purpose of the head lights is to focus or show the light for larger area as much as possible and to send a beam of light infront of the car. Because light from a source comes in all directions (diffration) and in case of head lights we don't want to face the light towards us. And we want to change the direction of those rays which are coming to us to travel along the road. This problem is solved by placing light source or bulb at the focus of the concave mirror because the light rays coming from focus towards the mirror after reflection goes parallel to principal axis or along the road.