Your battery does more than just provide electrical current. It also shorts AC, spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, potentially destroying every semiconductor circuit in your vehicle. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and the vehicle’s stereo as well. I’ve never been struck by a bus but I’ve never stepped out in front of one either. Some mechanics still do this little feat and some are lucky and some…not so much.