Our microwave oven has an "on" button, for whatever reason. Not a button that starts the microwaving, just one that turns on the display and enables you to push the other physical buttons. If you turn it on and then don't press anything else for a certain time, it will angrily beep at you.

They could just as well have skipped this and made it turn on when you press any button, and then turn off after a certain time of inactivity.