Why? Use to be able to tune my own vehicles for maximum fuel economy and performance, these two work hand in hand. Adjust the carburetor jets for a 14.7:1 AF ratio, and the spark advance to maximum without ore-ignition to gain the longest combustion cycle.
Yet another factor is compression ratio, the higher the better, one reason why diesels give better fuel economy, with direct fuel injection run about 20:1, 11:1 is about tops with gas. With low octane, get detonation, as the piston goes up, creates heat burning the mixture way before that spark plug does its job.
With a turbo, maximum CR can only be about 8.5:1, but with the Cruze, can jam in a lot more air into the combustion chamber that effectively does the same thing, peak pressures are as high as 35 PSI, much greater than atmospheric that at altitude can be as low at 10 PSI.
The O2 sensor keeps the AF ration at 14.7 and the anti-knock sensors produce the maximum spark advance, so with the new stuff, to gain extra performance simply by pouring in higher octane fuel.
Another way to increase octane is to add ethanol, start off with an 80 octane fuel and squeeze it up to 87, but not without problems, ethanol and gas to not chemically combine to form a homogeneous fuel. Ethanol is heavier than gas and first problem is getting a proper mix from the pump, second, when the car sits, ethanol will settle to the bottom of the tank where the fuel pump inlet sits and some of that ethanol is used first, leaving 80 octane fuel left.
Should be instructions to shake your vehicle first before starting it. Green people don't know anything about basic science.
Another objection to ethanol, its hygroscopic, holds moisture that causes corrosion and also attacks plastics. A true contradiction is you can only use E10, but not E15, both contain ethanol, only advantage of E10, takes a bit longer to cause fuel system problems. Ha, worked with Japanese, said Americans are stupid, no argument here from me.
Worse part of ethanol is using our food to power SUV's, can't buy a pound of hamburger for 50 cents anymore nor a pound of cheese, more like 5 bucks now! Forget about a New York strip steak.
You want reasons, learn something about the facts.
Gas is not very good either, HC's, H is okay, but its those C's that cause a lot of well over 100 year problems, very low heating value, a gallon is about 5.5 pounds of carbon, fouls your valves, blocks your fuel injectors, shorts out your spark plugs, and requires frequent oil changes and pollutes the air we breathe.