Fuel Injection tuning is required when engine modifications exceed factory system components. Some systems can be faked into running correctly by several devices on the computer and machanical side, however to properly tune you need to access and change the computer.
They have a given set of task and a given set of data associated with completing those task. When the data that the computer has is incorrect, the computer is in able to control the engine fuel delivery accurately. For example if your computer is programmed to run a 5.0-liter engine and you increase that to 5.7 liters, the computer does not know unless you tell it.