Cheap and dirty time travel, able to access areas the TARDIS can't. Though that would also mean the Doctor would be able to go back and see them using it, but it only works on one person so he wouldn't be able to take them anywhere.

None of that explains why the Doctor couldn't just take the TARDIS and go to, say, 1940, clear of the 1938 temporal disturbances, and pick them up a couple years later for them, and bring them home. He can't double back easily on his own timeline, but they're not on his timeline anymore ... except for the fact that he saw their tombstone, and if he went back and got them there would be no tombstone. That might be it. Going back to get them would alter his own past (his viewing of the tombstone), and create another paradox.

But overall, it'll be a cold day in hell when Doctor Who has firmly defined rules of time travel. I loved the episode.