It is circuit. The power comes from the battery and has to go back in. If you don't have ground you can't complete the circuit. Without getting all complicated about it the negative is the frame and engine block. If you look at the spark plug you will note the one contact that you adjust is connected to the threads. The threads complete the path to the engine block and then if you have a ground strap to the engine block back to the battery you can complete the circuit.
