Why A fuse contains a thin metal element that melts and opens the circuit when current exceeds its rating, cutting power before wiring or components overheat and start a fire. It is an overcurrent protection device, rated in amperes, and it is sized to protect the wiring and equipment behind it. Circuit breakers do the same job but can be reset instead of replaced.
Watch out The idea that a fuse limits current to prevent shocks is tempting, but a fuse does not limit current, it interrupts it, and it blows at levels far above the few milliamps that are dangerous to people. Ripple is handled by filter capacitors in the power supply, not by a fuse.
A fuse does not limit current, it quits. Too much current, it opens and the power goes away.