Скайбоксы(англ. Skyboxes) - solutions intended to make maps look bigger than they are. Usually used to display unreachable areas, such as distant mountains or buildings.
There are two types of skyboxes in the Source engine:
Consists of a giant cube surrounding the map used for applying textures in its interior faces. These textures usually consists of the ground meeting the sky in the horizon, clouds, and other distant details. The 2D skybox for a level is set in the Map Properties dialog of Hammer.
This is a newer addition for skyboxes in the Source engine. It's a surrounding area constructed by the level designer, outside the bounds of the gameplay area on the map. Players can never reach the 3D skybox. Maps do not require a 3D skybox to be created — it is a purely optional visual addition to the map. See 3D Skybox for more information.
Drawing a large box around your map, and making it hollow is convenient, however it is not so convenient for the final version. A huge sky box will slow down compiling, and loading.