Since Lion (10.7) it hasn't been possible to use your external display as the primary and have the internal laptop display shut off. This "feature" was removed in favor of the so-called "Clamshell mode" where Apple decided that the only way to turn off the internal screen should be to close the lid.
Why would you need to open the lid, if it's off?
There are several reasons. Some of them are:
1) Performance. The GPU is burdened by 2 active screens when lid is open. Even if you are only using 1 of them.
2) Wi-fi signal. Worse when lid is closed.
3) Heat. Will overheat more (and fan will come on) and worsen the life of your computer when the lid is closed. Opening it greatly increases airflow.
4) Digitizer sensitivity. Digitizers cursor will span horizontally over 2 screens, but vertically over 1. It will become more sensitive to minute movements and cause you to drag stuff when you meant to just click. (Personal issue)
How to fix it
Instructions are simple: Close the lid. Launch the program. Open the lid. Press escape.
(To re-activate the display, Close the lid. Wait 5 seconds. Open the lid.)
The source code is also open.