Practical Role
The Lunarians have a stockpile of enormously powerful & dangerous magical weaponry & utilities that could turn the Grand Disc on its head if even a handful were brought back to the surface. Going to the Moon & coming back is something that Lv10+ PCs would do in order to supercharge their conquests with world-wrecking power at the risk of bringing down the wrath of aeons-old proto-gods onto their heads.
Alternatively, much of the Moon is not actually under Lunarian occupation as it consists of a bizarre & deadly alien landscape with a variety of interesting rewards that by themselves could be extraordinarily valuable even if they didn’t change the fate of empires (merely kingdoms).