I'll explain the Visibility process of HL since I've nothing else to do except organize music, and that sucks.
Let's take a simple box room. We'll assume it's sealed from the black void. Now, since you can't see outside this box, our player will only see the four walls of his room and the ceiling and floor faces, thus rendering 6 faces. The other faces, exposed to the void, are discarded during the compile process since hey, they're never going to be seen. We'll mess with VIS's thinking shortly ;).
Let's add a nasty skybox, and remove a wall section for a lovely window.
Thus, the skybox is the box enclosing our 'box' from the void and thus IT'S outside faces won't be rendered, but the faces of our box-room will be rendered since they're within the skybox area, and presumably, will be seen by the player. Therefore, we'll see 4 extra wall faces, plus the side faces of the walls, ceiling, and floor. And that's if you've laid it out correctly.
Might not seem like much now, but once your map gets very complex, this will be a nightmare for VIS and the player too, since it'll make your compile times unnecessarily long and give the player unnecessary lag.