The camera transmits the point of view of an object in the scene. It basically takes the place of looking. If we are filming two people talking and we want to pass the point of view of one over the other, the camera will be in front of the photographed object in the place of the one we want to emulate his eyes. Usually the photographed object will not look directly into the camera, but 30 degrees to the side. If the object looks directly into the camera, it’s like he is looking directly at the audience and thus breaks the magic of cinema. Of course there are quite a few films that do this on purpose, but this should be a conscious decision.