Even when an object is within the field of view, it may still not be visible to the viewer -- for example, an object located behind a closed door. These have to be eliminated from the object rendering list.
Hidden surface removal could also be done later in the rendering process (during rasterization), by just having objects closer to the viewer overwrite objects further away. However, this wastes considerable computation in unnecessary rendering of these hidden objects. It is cheaper to identify and remove them earlier in the rendering procedure, when the work is proportional to the number of objects rather than the number of pixels.
For polygon-based rendering, hidden surface removal can be done very fast by just culling the back-facing polygons.
