After all - without considering EDGE and GZDoom right now - Doom is still a 2.5D game and therefore very limited. But with a clever usage of sky windows (and sky boxes as well) you can make your map not attracting the player's attention on this restriction.
Just think about many old and new maps you have played before where you have thought: "Yes, another map that shows dooms limits!" but also think about Scuba Steve's Action Doom that is full of tricks that make you think that doom actually became fully 3D!
One of the simple tricks you can always use that achieve these "I am in a 3D world" effects are sky windows. Openings in walls, floors and ceilings (or more of them at the same time) that are just there to make the player see what's beyond this room.
Take a look at both shot one and shot two. Imagine how they would look like without the openings and now see what they affect. The rooms get larger and also more realistic, as it feels more 3D instead of limited 2.5D. The power of these additions is obvious and don't forget: With the right skybox you can use them everywhere: Space Windows, Bull Eyes in the Sea, Atmosphere, Floating Hell, Burning Cities... you're restricted only to your imagination!
Summary: "Sky Windows are powerful as they are one of the things being able to create a certain reality 3D feeling!"