Recent Home Inspection I found the garage door sensors mounted on the wall of the garage!  The reason for these sensors are to sense if anything especially children, pets or items are in the way of the garage door when coming down. Photoelectric eyes. These eyes (also known as photoelectric sensors) are located at the base of each side of the garage door and emit and detect beams of light. If this beam is broken, it will cause the door to immediately reverse direction and open. For safety reasons, photo sensors must be installed a maximum of 6 inches above the standing surface.

From Garage Doors and Openers – InterNACHI http://www.nachi.org/garage-doors-openers.htm#ixzz367R9i7p2