I use the variable volt Darwin which is one of the top three VV on the market and in many respects the top one. Comparing a good VV PV to say a regular 510 is like comparing a Yugo to a Lexus. First, the sweet spot for vaping on a regular att or carto in volts is 4.5 to 5 volts. The small little battery models are 3.7 volts but actually 3.2 volts under load. The warmth of vapor on a VV or even a good 5 volt such as the GLV2 is just night and day better than on a dinky battery model.
Then, as mentioned above, you have the flexibility to use any atty/carto with it's different vaping characteristics at it's best vaping level by changing the power level of the PV.
Another great advantage is consistency. The Darwin uses "regulated boost circuit" technology. With a regular PV, as the battery becomes weaker, the vapor and warmth diminishes quickly. With the Darwin, the last inhale is exactly the same as the first inhale. And the Darwin goes 21+ hours on a charge for a heavy vaper. The Darwin also senses the resistence change in the atty as you vape and automatically adjusts the power output to the atty to compensate for the change in resistence.
If you like consistency, warm vapor, great battery life, great throat hit and flexibility, then a good VV is the way to go.