I've used both variable voltage mods (Provari) and variable wattage mods (
dna). From what I can tell, they work pretty much the same way. They allow me to adjust my vape "on the fly".
Most manufacturers of drop in
coils print their recommended wattage range on their
coils. If you're still using variable voltage you're out of luck unless there's a watts to volts conversion chart that I'm not aware of.