You might not ‘toggle on and off all the time’, but just not want a contactless card. Other banks (e.g. Lloyds I know for sure) will supply a non-contactless card on request, but if it’s configurable in settings Monzo would only need to ship one card, and not have to deal with such requests.
One might toggle online payments off for a while, until next needed say, after having to supply details over the phone - but still want to be able to use the card for ‘card present’ transactions.