The light position can be relative to the object or to the viewer. When looking at a small object in your hand, you turn the object and do not move your head, so the light stays in the same position relative to your eye. However when you move an object in an modeling program or when you look at a large part, the view point is actually moving, not the object. For example, two objects in a scene may be lit differently to draw attention to different attributes of each object. If this was accomplished by adding two lights to the environment, the multiple highlights could be confusing.
Most material properties are semi-constant as the view direction or lighting changes.
When a real metal part is rotated in your hand, the banding does not stick to the object, but remains constant since the environment is not changing. However, in an non-photorealistic interactive environment it may be too jarring to have the metal shading changing abruptly. Model with cool to warm shading with lights positioned up and to the right. After moving the object instead of the camera, allowing the surface to vary completely from cool to warm. Another notion is to allow the relative size of the object to control the motion of the viewer, the object, and the light source in an interactive 3D illustration.