I believe the Erdstall had a psychological and a physiological purpose. It fulfilled a psychological wish to be enclosed, and a physiological need for carbon dioxide. Our bodies produce their own CO2, of course. We have been gifted by evolution with many ways of getting rid of excess carbon dioxide. They serve us well when we are young and active. As we age, we become less active, so we don’t produce as much CO2. The mechanisms that get rid of it are still functioning, though. The Erdstall trapped its visitors’ CO2, and they were thus able to stave off many of the allegedly inevitable effects of aging.
Not everyone could take advantage of them, however. Some people were afflicted with claustrophobia, and were simply incapable of entering the Erdstall. The watched the Erdstall users remaining robust as they aged and sickened.
The twentieth-century’s most notorious genocide was originated by people from Bavaria and Austria. Perhaps they had done it before.