Until El Capitan, OS X/macOS doesn't have a friendly UI to disable the built-in display while an external one is attached. All you can do is fully decrease the built-in display brightness, but even though there's apparently nothing being displayed, if you look very closely you'll notice that the system is still rendering image on the built-in display. Yea, I can use in clamshell mode, but I'd rather not let the heat created by the chassi internals hit the display. I know Apple recommends using this clamshell mode, but I still think the user should be free to disable the built-in display whenever he wants. The brightness trick isn't very good because it still makes the GPU use it's processing power to render useless image on a display. It indeed consumes processing power, as you can read this testimony: https://discussions.apple.com/message/30390201#message30390201.