As stated on the wiki, it is used to (prevent the creation?) of visleafs inside its volume in case there are lots of props forming a symmetrical surrounding. However, I've only seen it being used in Lumberyard, or at least the only map I can recall right now that makes use of the entity inside the 3D Skybox. I tested it myself and it Does prevent visleafs from being generated but, in terms of performance, I noticed no difference in the map I'm planning to implement it (since its 3D Skybox pretty much resembles that of Lumberyard).
The 3dskybox is already automatically func_visclustered, VVIS merges every visleaf inside the volume that a sky_camera occupies
Like I said, I tested my map With and Without the volume on the 3D Skybox, and noticed that there are no visleafs when the viscluster is in use. Valve used it on Lumberyard, and perhaps on other maps too.
It makes no difference, the entire 3d skybox is rendered regardless No idea why Valve used it, since func_visclustering is already automatically done in the 3dskybox
