A mosfet is used when you are using a low amp switch and wires to conduct the voltage, and the low amp switch and wires may melt if the amp drawn is higher then they can handle. Example is a parallel plastic box using two 18650`s but with a 20 (or less) amp rated switch and wires. While it may handle most .25 builds fine, it might struggle and get over heated once you drop below .25 or so and start pulling more then 20-21 amps. Its a simple circuit breaker that just opens the circuit creating a dead connection, If it gets too hot. IN this example, without the mosfet, the switch and wires could melt down and cause the battery to go into thermal runaway because of the resulting short the melt down would cause.
IN a fully mechanical set up, all metal contacts, including the switch, are much larger and are easily rated for the max amperage and so the only thing you have to worry about is pulling too many amps and your battery melting down, ie thermal runaway. You can use a Mosfet in a fully mechanical box, but you would be limiting it to the operating amperage of the mosfet instead of the max amperage available of the battery's.
