Dr. Strangelove kicks off with a booming, third person male narrator voice giving us some info about rumors of a Doomsday Device. Shortly after that, Mr. Announcer Guy gives us the info on America's squadron of nuclear-bomb-carrying B-52's that are awaiting orders, ready at any moment to incinerate the U.S.S.R.
The narrator's main purpose is as an information tool, which fills us in on some stuff the filmmakers think we ought to know. Overall, it gives us the feel that we're watching one of those newsreels that used to come on before movie features back in the day. It creates a mindset in the audience that the out-of-control events they're about to see are grounded in reality. Nukes exist. They've been used before. There are still tens of thousands of them out there. They could be used again.
After he's done with his first two announcements, the narrator disappears and never comes back. Maybe, he got locked in the bathroom or something. Or maybe the filmmakers got what they needed from him at the beginning and weren't really all that concerned with his unexplained disappearance, since the world of Dr. Strangelove is so absurd anyway.