You wouldn’t drive your car without a spare tire, would you? When you blow a tire and you have to get where you are going, you will be glad that you have that extra tire in your trunk. When you run out of gas in the middle of nowhere, it’s too late to think about filling up your extra gas can.
The same goes for robot parts. If you begin thinking about ordering parts for your robotic system when the parts break, you’re already at a disadvantage.