For those who use their ATV 4K as a hub with HomeKit, I think the device has to be ON. If so that would be a reason to leave it ON 24/7.
Personally I have my home entertainment system set up to turn ON the ATV 4K whenever I press the Power button on its remote, as well as turn ON the TV and the sound system. I also have it set up to turn everything OFF when I press the Power button to turn the ATV 4K OFF. I don't like to waste energy, plus it doesn’t take that long to turn ON.