Correct me if I'm wrong on this. ^^
Most laptops has a function key (Fn+F8 on Dell)to switch the display devices. When you press the keys, it will show the display devices if you have external display devices plugged in, keep pressing the keys until the desired device is highlighted (selected), that's it. You have an option to show on both laptop monitor and an external display device, or either one of those. Control Panel setting is useful when you have more than one external display (LCDs).
It's the same as when you connect the laptop to a projector for a presentation/demo.