This would be needed for testing an add-on. You open it, but it would open a Blockland window with only a "Start a Game" option. Then a little window would open, where you could choose one add-on to run. Then, you press "Start a Game" and then it will start a single player game at Bedroom, loading nothing else than the add-on you chose. This is useful if you've made an add-on and when in-game, you see he's holding it the wrong way, and you don't want to start Blender/Milkshape and fix it and then start Blockland, etc. etc.
Long story short
A Blockland window that where you start a game, choose 1-3 add-ons and then starts a game at Bedroom, loading nothing but the add-ons. (Hammer, Wrench, Printer and the default bricks will still be enabled.)