as of the 1968 model year, all cars and light trucks sold in the U.S. were/are required to have front and rear marker lights. amber in the front, and red at the rear. the idea is that this makes the vehicle more visable at night time. some manufacturers choose to make them also flash with the turn signals, while others don't.
in some states they have to be working in order to pass a yearly inspection, necessary to license the car.