The shaders folder

[Jaws2002]

The reason you have those "shaders" in the cache folder is so that next time you fly in the same area, the game doesn't have to generate all those shaders again. Just grabs them from the cache folder.
Is good idea to clear the cache after you change some graphics settings so the game can generate new textured based on the new video settings.

I don't delete the skins from the cache unless something about how skins are handled changed. I don't want the game to download all of them again when i go online.