[malingo]

Same reason why all road vehicles have brake lights. Too difficult to detect the (decreasing) speed of a vehicle in front of you.

[glaberficken]

Its because of this that road authorities usually place "mandatory head lights on" road signs on long straight single lane pieces of road. (because if there is high traffic there will be people taking advantage of the long straight to overtake slower cars, and during the day there is the possibility that a driver will not interpret a head on collision correctly)