Overcurrent protection of conductors is an idea so simple that it is almost self-evident. Excessive current in a conductor results in rapid temperature rise, which damages insulation and creates enough heat to ignite nearby combustible material. Therefore, if you limit the current to a manageable level, you’ll protect the conductor and any materials nearby.
You can achieve this in one of two ways:
1) Insert a short segment of material of lesser ampacity (e.g., a fuse) in the circuit path, which would melt and open the circuit path before more widespread damage could occur, or
2) Incorporate a mechanical device into the circuit (e.g., a circuit breaker) that would perform the same protective function, but that could be reset any number of times rather than being replaced.
Fuses provide perfectly competent overcurrent protection. However, in residential and other low-power applications, they are considered obsolete, though still Code-compliant. Besides being resettable, a circuit breaker provides a clear visible indicator when it has switched off after detecting a fault. In addition, these types of panelboards are easier to wire and repair.