I'm not an electrical engineer but my guess would be to reduce the voltage drop. If there was only one ground, the longer wires would have a larger voltage drop than wires close to the ground. This can impact sensors and circuits that need a minimum voltage to work properly. Also for redundancy - if one ground degrades, there are other grounds that can pick up the slack. There may also be a reason related to timing and keeping all the ignition/fuel pulses synchronized. Just some of my theories...