Not every coil has the same resistance, cartomizers vary from manufacture to manufacturer, some juice is great hotter, some better cooler. Lots of variances.
A true non-VV battery comes off the charger at 4.2-4.3v, they instantly drop to 3.7v and hold there a while, then start dropping again. We used to have to find a juice we liked, then an atomizer that would work well with that juice, with the battery we were using. Then cry in vain when one or the other was out of stock and have to start all over again.
VV not only allows you to find the sweet spot with a juice you like, but say you can't get the carto's you want from the vendor you love? No problem, get the ones you can and adjust to find a new sweet spot. We are no longer limited by out battery anymore. Set it where you like it and most of the better ones will hold that voltage until the battery is dead. Change out the battery and go again.
It has made vaping, and dealing with the inconstantancies of suppliers, a lot more enjoyable.