When your map is compiled it is decided in compile time what parts of the map should be drawn in the game when you are standing in a certain position.
Doors are special because if they are open you can see the things behind them, but when they are closed you can't. Since doors are "see through" for the compiler, the game will draw things behind closed doors too. You need area portals to make doors behave like they have to.
I hope this is clear enough and don't hurt anybody who is not a beginner in using area portals.