the seed viewer refers to them as "mountain wells" and their swamp counterparts are called "swamp wells". I can't say what they are to the devs. the drauger are walking corpses of some of the folk that lived in valheim long ago so I just assume that they built and used them. there are plenty of real world examples of large wells with staircases in them.
I'm totally gonna see whats underneath them now! never knew there was more to that story