Fusilier,
The purpose of a plug cutter is to create a piece of wood that exactly fills in a hole in wood. This can be for a cylindrical dowel or in this case, the recess above a countersunk screw. Once you've glued the plug in, the screw beneath is completely hidden (but also very difficult to remove). Because different manufacturers of countersinks use different diameters and bevels, some plug cutters work better than others for filling in the hole and getting a perfect glue-ready joint. Hence the question on this specific application.
Hopefully this answers your question without sounding arrogant or "better-than-thou". That wasn't my intention. Just sharing my $0.02.
-Brandon