As with all electronic variable Speed setups you will loose Horsepower as you slow the motor down. This can stall the motor as you try to put some load on it.
A mechanical gear setup will allow the motor to run closer to its rated rpm where it can supply the horsepower as stated on the nameplate of the motor.
So when running slow the lower gear will give much more power to drive your tool.
Of course some tools do not require much power and will run fine even with the higher gear setup.