Depending on the user session software running, idleness can have multiple consequences, including, but not limited to:
Screen backlight dimming
Screensaver animation replacing the desktop content
Instant messenging client going to "auto-away"
Computer suspending or hibernating
Inhibiting idleness is supposed to stop all those actions from taking place, specifically in response to idleness. A user action asking for any of those states would obviously be acted upon witout delay.