A higher grade of petrol has more additives to clean the fuel system and injectors, and you will get more MPG to offset the higher price (also the higher octane rating means that the combustion doesn't attract the attention of the anti-knock sensor, which will retard the ignition if it detects certain frequencies of noises from the cylider head area - this reduces engine power). My jazz 1.4SE loves V-Power or Ultimate fuel and feels quite a bit livelier, some of this effect last for a while after I fill up with normal (95RON) fuel and could be due to the fuel system / injectors being cleaner.
Supermarkets reduce the price by cutting down on additives, my wife stubbornly filled up with this cheaper fuel until I sneaked a couple of tankfulls of 'normal' petrol in and she felt how much better her car ran.