ECU tuning can provide the car owner with a more enjoyable driving experience. It provides more power from the engine and better turbo response. This means that when you put your foot down, either at the lights or while already moving, you will enjoy a quicker and better response. Your car will accelerate quicker and you can overtake safely and quickly. It will remove flat spots from the whole rev range so that you can enjoy this responsiveness whenever you want it.
There are other ways to benefit from this too. If you continue to drive in the same way as before you had your ECU tuning done then you will make gains in your fuel economy too. Motorway cruising mpg can improve by more than 20% while town driving can enjoy similar gains if you use the improvements wisely. This means you could save as much as 15% at the pump every year.
ECU tuning is not possible on every single vehicle but is widely available for naturally aspirated and turbo petrol cars as well as turbo diesels. Turbo diesels are the ones that tend to enjoy the greatest benefit from ECU tuning but it is possible to find out likely gains depending on your make model.