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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.
 
Mine (the 2nd gen from 2010) acts as a screensaver when idle, so my TV and ATV are both left on. Makes for quite nice ambience since I'm using the National Geographic screensaver. Both the TV and ATV are from the same year and seem to be working fine left on 24/7.

FYI even when 'off' it's not truly off. It's in sleep state. Even homekit will function since the wifi is left active only the video output and front LED is off. The device is still running in the background.
 
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