Steam only lets one device play at a time, so two things can interrupt your idling. Auto-Restart handles both for you.
You start a game on your PC. Your account gets bumped off here. We mark it as paused and keep checking. The second you close your game, it goes right back to idling the same games as before.
Steam goes down. If Steam has an outage or restarts for an update, we wait for it to come back and reconnect on our own.
Either way, you never restart anything by hand. It's on by default on every plan. Want accounts to just stop instead? Turn it off in your settings.
For the complete setup and the limits Steam applies to simultaneous sessions, read our guide to idling Steam games with your PC switched off.