An ECU also known as an Engine Control Unit is found in every car. It’s basically the brain of the engine and uses sensors to measure factors such as oxygen content of exhaust fumes, the temperature of the engine and other measurable parameters. Using this information it helps control how and when fuel is added to the engine giving a balance between fuel economy, performance and wear and tear. This can be quite a complex process depending on if your driving at 70 mph on the motorway or are in 20mph zone or are doing a complex manoeuvre.
Each manufacturer basically decides the vehicles ECU parameters and sets them at a factory set level which can have no bearing on what’s best for the driver. Compromises have to be made for meeting emissions standards that may not even be applicable in your country! Or extreme temperatures and altitude driving etc. Some manufacturers even limit the vehicles power output purely to suit fleet purchasers of vehicles and for no other reason.