A fuse box is an important electrical safety device designed to serve as a first point of failure in an electrical system. Fuses are intentionally designed to fail at electrical loads below the capacity of the circuit, ensuring that any overload damages a replaceable fuse instead of causing wiring damage.
Any given circuit's wiring can handle a specific amount of current without problems. When current exceeds the normal load of the wiring, the wiring heats up due to electrical resistance. In a circuit without a fuse, a current overload eventually heats the wiring up to the point where it could start a fire or cause damage to equipment connected to the circuit. A fuse is designed to fail before this point to protect both the wiring and the more valuable equipment on the circuit.
Fuses are rated for their maximum amperage, and should only be installed in circuits rated for that amount of current. For example, if a fuse rated for 40 amperes is plugged into a 15 ampere circuit, it can allow the circuit to overload before failing, potentially causing damage to the wiring.
Most homes built after 1960 have reusable circuit breakers instead of fuses. These switches are designed to snap open when current levels exceed their maximum ratings, and can be reset to reconnect the flow of electricity.