Atlantis sells books!
In Brad Steiger's case, "Atlantis" is used as a catch-all term for "mysterious things from the past." These are ooparts, "out-of-place artifacts," ranging from seemingly too-advanced technology to a relic far removed from its origin. A Chinese coin found in an Indian burial mound would be this type of thing.
For Muck, the answer to ooparts is simple: There really was an Atlantis, who spread its worldwide civilization here and yon, leaving tiny traces behind. Muck even pinpoints a date when the asteroid hit the mid-Atlantic Rift, sinking Atlantis.
John Michell's ideas are a bit more "cosmic," implying that most of our physical environment has been shaped by those Titans of old, to make all the earth-type energy flow more freely and enhance man's spiritual life.
We can look around and tell that this didn't work out well, huh?