One theory says the increased comfort from "softer" tires may translate to less fatigue and more efficiency over distance.
That's the only theory that seems to apply to my real world riding. Along with improved bike fit and physical conditioning.
Recently I've swapped my low profile wheels with 700x23 and 700x25 tires at reduced pressure (usually 70-85 psi front) to a semi-aero front wheel (30mm profile, flat blade spokes) with 700x20 tire at 120 psi or more. It's a bit harsher and more tiring over longer rides, but may be offset by slightly improved aerodyamics.
The aero wheel and skinnier, harder tire is faster on a couple of 5 mile loops and over 30-50 mile rides but only on smoother roads -- mostly new or recently repaved access roads next to highways. Little traffic and few stops at night so it's easy to compare. But that same wheel and harder tire is a chore to ride long distances on chipseal and rougher roads, which eliminates the slight aero and rolling resistance advantage.
Too subjective to be useful to anyone else.