Here's why:

* Voltage rating: The voltage rating of a fuse indicates the maximum voltage it can safely handle. Using a 250V fuse on a 125V circuit is technically fine, as it's within the fuse's rating.

* Amperage rating: The amperage rating of a fuse is crucial for safety. It determines the maximum current flow the fuse allows before breaking the circuit. A 15 amp fuse will allow up to 15 amps to flow through the circuit before blowing. Using a 15 amp fuse on a circuit designed for a lower amperage could lead to:

* Overheating and fire hazard: If the circuit draws more current than the fuse is rated for, the wires could overheat and potentially cause a fire.

* Circuit damage: The increased current could damage components in the circuit.

* Failure to protect the circuit: The fuse might not blow when it should, leading to dangerous conditions.

Always use a fuse with the correct voltage and amperage rating for your circuit. If you're unsure, consult an electrician.