It's super handy because when you're near your Mac, access is open. But when you walk away from your Mac, it automatically becomes more secure. And if there is no iPhone or iPod Touch present, it will default to asking for a normal user name and password.
Lastly, it can launch applications or AppleScript events when your go out of range, or come back. So you could have it close iChat when you leave, and open it when you return.