A very quick and not too technical explanation of variable voltage vs wattage:
Voltage is what controls the level of heat going to your coil compared to your resistance to create wattage (which is power). You set the Voltage of the device. So the same power will go to any device, any ohm. Let's say you have a 1.8ohm coil that works well at 3.6 volts, if you swap to a 2.5ohm coil on another device you will get a much different vape because the overall power to the coil has changed because of the added resistance. You would probably then up your voltage to maybe 4.3v to get the same kind of "vape".
For variable wattage you set it once and forget it (sort of) Unlike variable voltage you are setting the end result output. The Device takes into account the ohms and sets the output for you to arrive at the selected watts.
If you set your variable wattage mod to 8.0 watts two things will happen. First the mod will automatically detect the ohms of what you put on it, let's assume a 2.5 ohm cartomizer. Second when you fire your device it will look to see the ohm rating of what your using, and then adjust the voltage, in this case to 4.4 volts, then fire. Likewise, without changing anything, should you swap that 2.5 ohm cartomizer, for a 1.8 ohm atomizer, the device will automatically detect the change in the ohms of your atomizer then fire at around 3.8 volts.
This isn't to say you won't ever change the wattage, you will, depening on the coil type, the liquid or other factors but you will most likley not do it as often.
This is a very simple explaination.
Please note: this is for premanufacturered devices only, not for reference on using your own hand made coils.