*Depending on the type of wood and it's thickness, driving screws by hand would require strength and constant pressure on the tool to keep it from stripping the screw head. The weight of the screw gun helps keep that pressure on without as much effort. True that it makes the torture go faster, but it also lets one run the screw in and out multiple times.
There's also the trick of applying soap or oil to the screw too decrease the friction in the wood. I would go with soap however, as an oil lubricant may be toxic for breast tissue.
*From years of working with wood and screw guns.