Those 20 built-in textures have been specifically created for the purpose of providing certain texture colours for use with TransDEM (hence their name). They are provided to supply textures to the terrains created by TransDEM to show the numerous parts of topographic maps as needed by the program. You, personally, unless you have and use TransDEM or download a map created with that utility which requires these textures, will otherwise have no need for them. Having said that, I would not remove them if that is the agenda behind your question, as, without them, any route that you may download which needs them as dependencies will show nothing on the terrain, making it virtually impossible to navigate your way around it..