The point being made, of course, is that the brain conducts a huge amount of processing on images the eyes send it. It takes dot patterns and recognizes them as familiar objects, so your mind can manipulate those. It recognizes rotated objects, so you can recognize objects from different directions. Thus, most of your sense of vision is not in your eyes at all. It is in your brain.
People simply look too much alike.
Yet, this is a critical need in a highly social species such as we are. Something had to be done. It is this cop's job to route incoming visual data to the face processor if it is a face, and to the usual circuits if it is not. Since an injury or malfunction can occur in one spot and not the other, this situation establishes a physical explanation for the occurrence of face blindness. It turns out that the traffic cop described above does not know whether his individual is face blind.