Current flows along a single cable from the battery to the component being powered, and back to the battery through the car metal body. The body is connected to the earth terminal of the battery by a thick cable.
A circuit usually includes more than one component, such as bulbs in the lighting circuits. It matters whether they are connected in series one after the other or in parallel side by side.
However, a component with a high current consumption must not be connected using wires which are too thin, or the wires will overheat, blow a fuse, or burn out.
If the cable overheated, there could be a fire. The fuse prevents that happening, because its thin wire will melt and break the circuit long before the cable itself can heat up and burn. Some cars have only two fuses. One rated at about 30-50 amps protects components wired through the ignition switch — flashers, wipers, heater-motor and instruments.