FYI - In some situations you might have a metal cover plate over an
electrical switch and usually there are metal screws holding on a plastic
cover plate.

Sometimes there can be a stray wire touching the metal box, the metal on
the switch, or a metal cover plate. I suppose a switch could also
malfunction internally and cause the metal on the switch to become
energized.

If the metal box and the metal on the switch is grounded, then should a
stray wire touch either, or touch a metal cover plate, then the breaker
should trip turning off the electricity and preventing someone from being
shocked (Or re-direct the electricity to ground).

I've seen situations where someone was doing dishes in the kitchen, went to
turn on the garbage disposal, touched the metal screw, and was shocked.
(Person was OK in this situation, but wanted it fixed of course...)