Here are some of the reasons you might need that back-up. You can lose your ATM bank card. Sure, you'll take more than one, but they may all be lost or stolen. The machine may refuse your card. The machine may eat your card.
Some computers seem to have a temper.
The machine may dispense its last peso or dinar just before you step up. Be sure to ask before starting your trip. PIN numbers sometimes fly from the memory. And, of course, there may be no machine where you are. In my opinion, that's a very good sign.