Believe it or not, the US Postal Service has what is called a Mail Recovery Center. The old name of this service, however, is the Dead Letter Office. This is where undeliverable mail ends up, either because the address can't be found or for some other reason. Periodically, unclaimed packages will be auctioned off.
The significance of Bartleby's having worked at the Dead Letter Office is that he has become the personification of a dead letter. Every day he does less and less work until finally he is doing nothing at all, saying, "I prefer not to." His employer does fire him, but he just won't go away--because he has nowhere else to go. His boss has to move his offices to get rid of Bartleby, who keeps going to the same place he has always gone to.
Have you heard the old saying that you'd have to burn the house down to get somebody out of it? That's sort of like Bartleby. Eventually, the new owner of the office has Bartleby arrested, but he is so lost in jail that he starves himself to death, preferring not to eat.
Perhaps he serves as a warning to us not to get so wrapped up in our jobs!