Idle is used to put the device in an “idle” state. This is often used with a Light that you want to always return to a preset DIM level, after it has been turned to full brightness. Idle is restricted if the device is in an OFF state so you don't have the light coming back on after you turn it off.
Lets assume we have a light at our back door and it is connected to a MOTION device that will turn the light on to full brightness when it detects motion. We don't want to leave the back door light on at full brightness all the time so we tell it to IDLE when there is no motion for 2 minutes. The IDLE command sets the light level back to 40%.