High octane fuel was all about stopping the knock- it destroys your engine. High compression engines knocked loudly on low octane gasoline. So, the move to higher octane fuel started. One of the drawbacks of high octane fuel is it burns "slower" or "cooler" so the gas expansion is slightly less, so you get less energy out of the fuel. Octane can be adjusted by changing the length of the hydrocarbon chains in the fuel (more of the "lighter" molecules) or by additives (ethanol is a high octane).
So you pay extra and get less for your fuel with high octane, unless you have a high compression engine designed to run on higher octane fuel. Of course, this discussion is about the 87/89/91/93 octane fuels in the US, not the 67/75 octane crap available in some countries.
Some vehicles are designed to run on multiple fuel types. I don't know if octane ratings have any different effect on these engines.
Chuck
The Model Hobbit wrote: