Why do only some stages use the feeder while others get a box of screws from which they have to pick up by hand?
Some types of stainless are non-magnetic and thus can't be picked up with a magnetic screwdriver, thus the need to hand-feed the screw
Also, it could be in some stages if a single screw is not placed, it will lead to catastrophic failure of the product. (Hence the screw counter in that first stage shown)