I bought a fixed voltage USB passthrough as my second device after a very cheap china cig-alike. It seemed like a low cost way to experiment to see if I liked vaping. It puts out about 4.85 Volts which turned out to be too much for my tastes and I don't use it anymore. I also don't like being tied to my computer (or an outlet with USB adapter). I'm happier with around 3.7 volts from my egos. I have a handful of starter devices now that run at different voltages, from 3.2 up to 4.2, including an inexpensive box mod I built myself. It would have been easier just to start with an ego twist VV, but I do like to tinker with things so I'm still happy with most of my choices.
Voltage goes hand in hand with resistance of the atomizer coil to go from barely producing vapor (low voltage/high resistance) to lots of very warm vapor (low resistance/high voltage). Too high and you'll just get a burned taste or blow the atomizer coil. type of e-liquid changes the experience as well.
Everyone seems to have different opinions on what is best, so VV makes it easier to fine tune to what you like.