Standard electronic cigarette batteries generally work at between 3.7 and 4.2 volts. It’s what they do and you have no control over it. It just pushes out its power at the same rate. Voltage, if you're not sure, is the term for electrical pressure.
Variable voltage batteries allow you to adjust the power output, which means change the voltage the battery is working at and therefore the heat that’s applied to the coil. The coil is the heating element inside the tank.
The benefit of a variable voltage battery is when you’re not getting the rich flavour or the full throat hit you’d like and you wish you could crank it up, so-to-speak (or even turn it down in some cases).
You get to control the power by turning it up or down which will usually affect vapour production, flavour and throat hit.
Each e-liquid has a ‘sweet spot’, an optimal temperature that it should be vaped at for the best flavour, and variable voltage batteries give you the flexibility to experiment and adjust for these sweet spots.
In essence, Variable voltage batteries give you more control of your vaping experience.
If you're a relatively light to moderate smoker, up to 20 a day say, or you smoke light cigarettes, you may find the vapour production and/or the throat hit of a variable voltage device too intense at higher power levels.
If you have to turn it down to get your preferred vapour production and throat hit, just bear in mind you could also consider using a lower nicotine level in your e-liquid, which will reduce the hit but not so much the flavour and vapour.
Experienced users tend to get the best use from variable devices but if you're aware that it make take a bit of time to get it right there's no reason why you should be put off from trying.