Of course, Hey Jude is one of the first real clues we get in the Gunslinger that Roldand's world is somehow a mirror of our world, or at least deeply connected to our world. (I'm being vague since the book is vague at this point.)
It's a strange mix: Roland's world is one of cowboys and the Beatles. There is a feeling that great technology once existed here, but things have returned to a simpler time.
The Guardian article notes:
you can find it in 55 books, from Stephen King's Wolves of the Calla ("The people are real. You … Susannah … Jake … that guy Gasher who snatched Jake … Overholser and the Slightmans. But the way stuff from my world keeps showing up over here, that's not real. It's not sensible or logical, either, but that's not what I mean. It's just not real. Why do people over here sing Hey Jude? I don't know")The Guardian article is HERE.
In a November 17, 1988 interview with King, Janet C. Beaulieu discussed the Dark Tower quite a bit. Here is what they said about Hey, Jude and the Gunslingers world:
SK: I see the gunslinger's world as sort of a post-radiation world where everybody's history has gotten clobbered and about the only thing anybody remembers anymore is the chorus to "Hey, Jude."
JB: Yes, that keeps coming back. And there was another one - it's not coming to me.
SK: Well, it's a different world; it's not our world, but it's obviously a world that's been influenced by our world. There are some little funny islands of the past like Atlantis that are still there hanging around.
JB: The idea of a honky-tonk "Hey, Jude" is kind of neat.
(Beaulieu's interview is HERE.)