Mechanicals do not regulate voltage. You get the voltage that the particular battery or
batteries deliver. Basically you get either 3.7 volt or 6.0 volt. (there may be other battery configurations I am not aware of) As the battery drains, the power delivered decreases because there is no regulation. This is called "battery drain".
VV and VW provides "regulated" power, maintaining the power level you set regardless of the battery draining. And of course you can set the power level you prefer regardless of the battery.
VW also senses the resistence of the atty/carto you are using and
automatically adjusts the power to maintain your original power setting even as the resistence of the atty/carto changes due to use or because you switched to a different resistence atty/carto. VV does not have this feature.
There are some other differences, but this is the basics.