They were added as a result of a crash, if I remember correctly the aircraft involved had a flat battery, they used external power to start an engine, then started the other, there was something in the flight manual re charging the battery etc that was missed by the crew, the aircraft was running ok and they departed, however the battery was in essence still nearly flat but was supplying sufficient power for the instruments and FADEC, upon take off the electric gear was selected up, this drew the available power resulting in the FADEC failing and both engines shutting down resulting in a crash landing, hence the independent power source being added.