What do fuses and circuit breakers do?

Fuses and circuit breakers protect the conductors that are connected to them from not letting the current going above the current rating that is given to individual size conductors.
the main function is to quickly stop the power to continue threw the loop., and to protect ,equipment from damage and people from dying, that may be in contact to a shorted circuit

Fuses and circuit breakers are overload protection devices, designed to protect loads (e.g. appliances) against overload or short-circuit currents. They are not designed to protect people.
to over come the over load conditions.

In simple terms the purpose of a fuse or circuit breaker is to protect the wiring from overheating and causing a fire. That is why it is so very important to match the fuse or breaker with the size wire in the circuit.

Protect the circuit from over current. Fuses and breakers prevent excessive amounts of current to flow through the conductors in a circuit which causes heat to build up making for a fire hazard.
A circuit breaker or fuse is a protective device that shuts off power when the current exceeds a preset limit. It is designed to protect the wiring, and thus prevent damage to the equipment and fire.