In "Bartleby the Scrivener," what is the significance of the discovery that Bartleby had been a subordinate clerk in the Dead Letter Office?

The lawyer who narrates the story has long been puzzled over Bartleby's behavior. Why did his scrivener, whose job it was to spend his days copying out and then checking the accuracy of legal documents, simply say "I would prefer not" anytime he didn't want to do something? Why in the end did he prefer to do nothing? Bartleby himself never got angry and never supplied a reason why he wouldn't work.
After Bartleby's death, the lawyer learns ("hears a rumor") that Bartleby had formerly worked in the Dead Letter Office. Here, in a low-level job ("subordinate clerk") that wouldn't have given him much control, he dealt with letters that never reached their destination, the lost mail that was neither received by its recipients nor returned to the sender. The lawyer speculates that it was the hopelessness of this job—he likens handling dead letters to handling dead men—that drove Bartleby over the edge to eventually decide to "prefer not" to do anything. As the lawyer puts it:

Dead letters! does it not sound like dead men? Conceive a man by nature and misfortune prone to a pallid hopelessness, can any business seem more fitted to heighten it than that of continually handling these dead letters, and assorting them for the flames?

The lawyer goes on to imagine the contents of these letters—engagement rings that never arrived to the beloved; needed money that never got into the hands of a hungry person—and the way miscommunication can lead to death:

Sometimes from out the folded paper the pale clerk takes a ring:—the finger it was meant for, perhaps, moulders in the grave; a bank-note sent in swiftest charity:—he whom it would relieve, nor eats nor hungers any more; pardon for those who died despairing; hope for those who died unhoping; good tidings for those who died stifled by unrelieved calamities. On errands of life, these letters speed to death.

The lawyer believes dealing with such hopeless letters led Bartleby to shut down and "prefer not to" go on. To him, it is the key to Bartleby's strange behavior.