It's for safety. It's been proven that the human eye will see the points of light in the general field of vision. So as you glance around you are less likely to miss an oncoming car and pull out in front of it, etc. Same reason for lights on a motorcycle since it's a small target (so to speak) and the recently added third brake light put at eyes level for the car behind you; you see it sooner and can react quicker. Of course it adds a bit of cost and the American companies have been dragging their feet adding it to all cars before the deadline which I think is the 2006 model year.