You asked for it so here is some more theory; The correct answer is referred to as stochiometrics (Gas stochiometry). To get what would be considered the ideal burn-off of all (as close to all as possible) fuel upon combustion, many important things must come into play. The correct air/fuel ratio, at sea level, of 14.7 to 1, the correct containment (cylinders;
and compression is important), timing, and of course the correct spark, all play a part in complete combustion. All of this is taken into account upon design. This would include the type of parts used as well as the gap between a rotor and the cap (if equipped), and the spark plugs. The spark "must" reach the mixture at the correct time. This is the dwell time (firing time from start to ignition) and is so critical it is measured in milliseconds and has to be uniform across the spectrum of all the cylinders. This is because all cylinders are connected by a common intake manifold, as well as things like a common E.G.R. valve, vacuum lines etc.., and improper firing on one can affect another.
The "grade" of fuel is taken into account upon design and a higher grade (hotter) fuel, that does not facilitate complete combustion, can enhance performance but might cause fuel economy to suffer and create more negative emissions. Using a hotter fuel may, however, create a more perfect environment for stochiometry and thus the complete burning of the mixture, creating more power as well as improved fuel economy.
In your case, if the vehicle was manufactured to use regular unleaded, and you see improved power along with improved MPG then it is possible that some of the parts in the system might not be functioning at 100% (worn or some wear) and the higher octane is what is needed to give a complete burn. This is just a theory off-the-cuff, and I might possibly have a better answer with study, but I am probably correct. As far as harming the engine, a complete burn-off of all fuel mixture is ideal and will not harm the engine. Bear in mind the mileage, age, and the fact that "hotter" can facilitate more wear in direct relation to the driver wanting to exhibit more "power", as in the more "pedal to the metal" the less life can be expected.
I was an automotive technician that studied hard to give my customers what they paid for which is great service so I had to know what was needed. I also believe that an engine diagnostic machine with a vacuum gauge and scope is vital to a properly performing engine so if you go to a shop without one (for a tune up especially) it is most likely a jack-leg operation at best.
You may find an answer you like better but you have to admit this is not bad for an ex-technician and current truck driver right?