Often simply called "the tramp," Jeff Allen is a man who has fallen on seriously bad times. His sole purpose in the book is to tell Dagny the story of the decline and fall of the Twentieth Century Motor Factory. Jeff Allen is sort of like Coleridge's Ancient Mariner. In The Rime of the Ancient Mariner, the Ancient Mariner wanders the earth telling his tragic personal story, almost as if he were compelled to. Jeff definitely seems compelled to tell his story to Dagny.
Jeff is also something of an Ishmael, like the narrator of Moby-Dick. The epilogue of what is essentially Ishmael's tale is titled with a quote from the Biblical Book of Job: "And I Only Am Escaped Alone to Tell Thee." In a way, Jeff Allen is the lone survivor of the wreckage that is Starnesville. He's held on to enough of his humanity, and his sanity, to impart his lessons to Dagny. It's significant that when he concludes his narrative, we don't hear him speak much anymore, except to tell Dagny his name (his very own "call me Ishmael" moment).
Jeff Allen delivers a lengthy, uninterrupted monologue of the events of the Twentieth Century Motor Factory, finally giving us a straight answer about who John Galt is and answering a ton of Dagny's long-standing questions. The Factory was sort of like the country in microcosm, and Jeff comments on the tragedy that occurred there, where decent people unknowingly destroyed themselves:
And if anybody had any doubts, he felt guilty and kept his mouth shut. Because they made it sound like anyone who'd oppose the plan was a child-killer at heart and less than a human being. They told us that this plan would achieve a noble ideal. Well, how were we to know otherwise? Hadn't we heard it all our lives . . .? (126.96.36.199)
Like many characters in the novel, Jeff discovered the awful truth about the morals he had learned all his life by living through the ultimate destruction those morals brought about.