High octane fuel is required when cylinder compression pressures are high enough that fuel ignition may occur within the chamber when the piston is on its way up; pre-ignition. Higher compression ratios = higher combustion chamber pressures = higher heat. At higher temperatures the fuel is burned more efficiently. So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process. Also, higher octane fuel reduces engine knocking or pinging. So, if you have a low-compression engine, save the $5 and use 87-octane.