THE imminent end, on May 20, of David Letterman’s 33-year run as a late-night TV host is a sad event for a number of reasons. Mr. Letterman’s “Late Show” monologues are still the gold standard in an increasingly crowded field. His Top 10 lists remain the most durable running gag in late-night television. (“Top 10 Things Overheard in Hillary Clinton’s Van”: “Polling indicates we should take the Taconic.”) He virtually invented the age of irony, and his self-mocking style has kept him a paragon of cool, even as his competitors have grown younger and his audience older.

But it’s easy to overlook the most important thing Mr. Letterman has nurtured in his three-plus decades as a nightly talk-show host: talk.

Talk — relatively spontaneous, genuine, unrehearsed conversation — was, of course, the main point of the genre when the “Tonight Show” was pioneered by Steve Allen back in 1954, redefined by Jack Paar when he took the helm in 1957, and turned into a national institution by Johnny Carson in the ’60s and ’70s. Here was a place where show-business celebrities could drop at least some of their public persona and give us a glimpse of what they were “really” like. Sure, that glimpse was always a little stage-managed — the conversational topics screened, the anecdotes carefully baked. But those nightly sessions on the “Tonight Show” guest couch were a relaxed, human-scale refuge in a hype-filled showbiz world.

Mr. Letterman, like Mr. Carson before him, understood this. He never shirked his publicity duties (“let’s show the clip”), and he valued guests like Martin Short and Steve Martin, who came primed with fresh material. But he took the interviews seriously. He asked real questions and actually listened to the answers. He rarely fawned, or let his guests off the hook. He poked their sensitive spots and cut through the phoniness.