Mainly because some people handload ammo for more than one rifle in the same chambering. Sometimes the chambers will be close enough in dimensions to handle neck-sized brass, but more often not. As an example, my wife and I own several rifles in .223 Remington. All shoot the same basic load well, but for the "same" load to work in all the rifles the cases need to be FL sized.
Plus, even if the same cases are always fired in the same rifle, after a few firings they often need FL sizing to chamber easily.
