In a nutshell, the factory ecu is set up to make the car work under all conditions, in all environments. In a nut shell, it's not the optimum set up, but the best average.
Imagine, the engine has to cope with top grades petrol to stuff full of crud in the Middle of Africa, from foot to the floor boy racers to Sunday drivers, to someone towing a boat.
By remapping, you optimise the engine to one extreme or the other.
