.5w increments are pretty close to .1v increments, but with VV the adjustment is a bit more fine, for example:
For wattage there are 25 total available settings, for voltage there are 31 (assuming your standard 3v-6v 3w-15w chip)
I usually use wattage when using carto tanks as 10w with cartos is my sweet spot, when using rebuildables I switch to voltage so I can fine tune a bit more. Plus, in wattage mode you can't get as much power as you can in voltage mode, using LR stuff.
For example, a 1.5ohm coil set at 15 watts is only delivering 4.7v. If you switch to voltage mode, you could turn to the voltage up to 5.5v for example, and get around 20watts, and still be well under the amp limit of the device. This type of power is only possible in dual-battery configuration, with a single battery the voltage increase will flatten out a lot sooner.