I believe a fuels octane rating is it's resistance for pre-ignition (pinging/detonation, marbles in a coffee can sound). The higher the rating, the higher it's resistance for pre-ignition/engine damage. It promotes a slower more efficient burn, as stated above.
Also octane booster in a can is a total waste of money. It is extremely diluted, even moreso when added to a tank of regular gas. It has very little benefits.
I think the reason it's not added more, is because the majority of cars are designed for the cheaper stuff, and the same old stand by oil answer "It's alot more expensive to produce". Higher horsepower cars, and cars running increased timing need the premium stuff.