The single most important use of a spare breaker is as an empty hole filler.

We see lots and lots of panel photos with empty gaping holes, waiting for curious fingers. This is where the cover knockout has been removed, and there's no breaker there, and quite likely, the official plastic blank cover plate fell off because they're flimsy. Spare breaker? Problem solved.

Plain breakers are an option for fewer and fewer circuits of late.

However, here's something: if you have a 240V load that does not have a neutral (so we're talking water heater, air conditioner, EVSE etc.) you can take 2 singles and sandwich them into a 240V breaker, using a factory handle-tie. Again this does not work if the circuit is 240V with a neutral.

See, common trip (one side overloads, both sides guaranteed to trip) is provided by an internal mechanism in a 240V breaker - not by the handle tie. That's what makes 240V breakers different from 2 singles handle-tied. And if a load is 240V with neutral, it needs common trip. But if it's 240V without neutral, it does not need that. So sandwich away!

If a circuit requires AFCI, you can run metal conduit to the first metal junction box, and use an AFCI receptacle there. (the metal conduit provides superlative protection for the wires from breaker to recep, thus, acceptable.) NEC 210.12(A)(5). My standard ploy is to fit a 4-11/16" square deep box right next to the panel, and connect it with short EMT metal conduit to the panel (thus taking care of ground). The bigger box has ample space for dual GFCI or AFCI receptacles with their wiring.

Plain breakers are an option for fewer and fewer circuits of late.

However, here's something: if you have a 240V load that does not have a neutral (so we're talking water heater, air conditioner, EVSE etc.) you can take 2 singles and sandwich them into a 240V breaker, using a factory handle-tie. Again this does not work if the circuit is 240V with a neutral.

See, common trip (one side overloads, both sides guaranteed to trip) is provided by an internal mechanism in a 240V breaker - not by the handle tie. That's what makes 240V breakers different from 2 singles handle-tied. And if a load is 240V with neutral, it needs common trip. But if it's 240V without neutral, it does not need that. So sandwich away!

If a circuit requires AFCI, you can run metal conduit to the first metal junction box, and use an AFCI receptacle there. (the metal conduit provides superlative protection for the wires from breaker to recep, thus, acceptable.) NEC 210.12(A)(5). My standard ploy is to fit a 4-11/16" square deep box right next to the panel, and connect it with short EMT metal conduit to the panel (thus taking care of ground). The bigger box has ample space for dual GFCI or AFCI receptacles with their wiring.

But the single most important use of a spare breaker is as an empty hole filler.

We see lots and lots of panel photos with empty gaping holes, waiting for curious fingers. This is where the cover knockout has been removed, and there's no breaker there, and quite likely, the official plastic blank cover plate fell off because they're flimsy. Spare breaker? Problem solved.