You need different voltages to aquire the same experience (TH, vapor production, flavor, heat) on different resistance attys.
If you are used to a lower riesistance atty at lower voltage, then change to a higher risitance atty (and want the same experience) then you will need more voltage.
A VV device lets us customize the experience.
In reality: For me I prefer to vape at 7.5w in the morning and as high as 12w in the evening. Without a VV/VW device, this is only possible by changing the resistance of my atty through out the day. With a VV/VW device, it is a quick setting change on the same atty.