lots of advantages to a VV/VW APV over a Disposable,.. its almost not right to compare the two.
But, basically, VW/VV allows you to set the ideal voltage or wattage, for the particular atomizer or cartomizer you are using. Some are low resistance, some are high resistance. a 2.0ohm Carto is going to need less volts than a 3.0ohm carto.
Furthermore, VV/VW allow you to go above the 3.7v mark, which is the standard ego voltage. I vape at a higher voltage than that even on my usual low resistance cartomizers. the only time I set a device to 3.7v, is the rare occasion I pull out my HH357 Atomizer, which is a 1.5ohm, and sometimes, juice depending, 3.7 is on the money.
ALSO, within the ballpark of each resistance/voltage combination, is going to be a grey area. I personally start a 2ohm Carto out at 4volts, but may go as high as 4.6, even 4.8ohms.. but generally settling between 4-4.5v. Why? Because every juice is different. Some juices dont give up their goodness until they are pushed to just before burning. Others are not tolerant of high heat at all, and burn at the thought of going any hotter.
the only time I would say a VV/VW device would be wasted on someone, is if someone didnt enjoy
vaping and had no interest in trying new juices and techniques etc et c, and was purely suffering through to quit cigarettes. If you enjoy vaping, even a little bit, or a hardcore enthusiast, a Variable Voltage device is one of the first things you should purchase. Even the simple Ego Twist is better that not having VV.