Reasoning:
You may want to make variants of a terrain with different terrain lighting (like BI did with Contact DLC for Enoch), or water/tide or any other configuration.
Currently it would expand the editor or MP selection. A scope version could limit that.
Missions on these hidden terrains would be still accessible via campaigns, SP missions, or scripting commands:
https://community.bistudio.com/wiki/playScriptedMission
https://community.bistudio.com/wiki/playMission
https://community.bistudio.com/wiki/hostMission
Note: Via small optional mod one could unhide a terrain for mission makers.