Every screw that's larger than it needs to be to do its job wastes money and resources.
Even assuming a large screw would work in the same place a smaller one would.
A screw that's smaller than it needs to be in a power mower will kill you.
That being said, a good designer will try to minimize the variations as much as possible.
I have to keep a set of metric and regular tools in stock. Half the stuff you buy today has metric fasteners.