Our posture changes during the day. Our tasks vary to some degree. If we have windows, light in the room changes from morning to evening. In response to these changing conditions, we need to be able to adjust the lighting levels directed on our work in order to maintain maximum performance. Task lighting allows us to correctly position the angle of light to eliminate glare and veiling reflections.
Magnification is only half of the solution for achieving good vision.
Proper lighting is of equal importance. Take an extreme example. The benefits derived from visual aids such as magnifiers and prescription glasses are entirely dependent upon the lighting conditions in which they are used. Proper task lighting allows us to get the maximum benefit from a visual aid, and may even allow for reduced magnification. Examples of poor contrast include faded print on white paper, or dark print on a dark background.