Control, consistency, longevity, flavor, density, and on and on. If a particular build and/or juice is best at 3.9v, why would I want to be locked into starting at ~4v and slowly dropping to 3.7v over a relatively short time when I could hit 3.9 every pull, all day? And if I like a build or a juice at 6v, then I'm really screwed with a mech. I've found it hard to go back to mechs, wish I could afford a decent regulated squonker! Also a good regulated mod with short protection gives me an ohm meter on the same hardware I tweak and set my coils on, so even if I'm building for a mech, I build on a vv/vw, usually a Sigelei Plus.