Crab, are you the Professor or Gilligan?
Saying that Octane doesn't have as much energy as gasoline is about like saying that H2O won't freeze at the same temperature as water.
That premium gas in your tank is 93% Octane and 7% Heptane (plus a volumetrically small amount of additives). These are both hydrocarbon molecules produced from "cracking" crude oil.
Heptane has seven carbon atoms and will ignite with relatively low compression.
Octane has eight carbon atoms and can be compressed much more without spontaneous ignition.
This is why a higher compression engine requires higher Octane content. This wasn't the case back in the days when oil companies were allowed to add tetraethyl lead to the soup. But, that's another story.
Why high compression engines? Well, in a normally aspirated engine, higher compression = more power all else being equal.
Your friendly Exxon fuel engineer blends the two molecules to get just the right resistance to detonation for your GTO thereby extracting maximum dollars from your wallet.
As for ethanol in cars, seems like a waste of good moonshine.