Why does stick up cam require battery if plugged in?

Just had a chat with Ring support and learned that even if I buy a power adapter for my Ring cam, it still requires the battery be installed. That doesn't make any sense to me. Can someone shed some light on why a device that's plugged into the wall also requires a battery. I wanted to repurpose the battery from the indoor cam as a floating spare to use on my exterior cams when their batteries ran low to minimize down time, but apparently, that's asking too much. Looks like I'll have to buy another battery *and* a power adapter if I want to do this. Not very happy about this. It seems like an intentionally engineered limitation. I can't think of any other electronic devices that require a battery when plugged into a wall outlet.