View Volumes and General Projection Transformations
You have to remember that the view volume is the volume which sets up near and far extents, top and bottom extents and left and right extents. These extents are chosen so that you can eliminate those objects from further processing that are not visible within the view volume. Any object with its defining boundaries beyond these extents will be immediately removed from the display list. For example, in a scene, there could be some very complex objects that are very far and do not contribute much to the scene. In the absence of a far extent plane such objects will remain in the display list and will consume lot of processing time. With these view extents well defined in the beginning, performance of display processing will be much improved.