A headlamp bulb, for example, is designed to have a degree of resistance so that it consumes a certain current to glow normally. But there are at least two headlamps in the circuit. If they were connected in series, electric current would have to go through one headlamp to get to the other.
This means it must be connected using wires thick enough to carry 4 amps comfortably. Often the power consumption of a component will be stated in watts, which are found by multiplying amps and volts. The lamp in the example consumes 48 watts.
Most car handbooks and service manuals include a wiring diagram which can be difficult to follow. The colour-coding, however, is a useful guide to tracing wiring.
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.