Even in perfect visibility its amazing how cars can merge into the background. One problem is they are normally just one colour, that's why emergency vehicles have stripes. Headlights provide the same kind of contrast that makes a car less likely to fade out of vision.

True enough, the light uses energy, and that ultimately comes from fuel, but the extra is minimal. silly not to really - that's why Swedish Volvos are set up to have their lights on all the time.