There is some critical debate as to whether Melville added the epilogic story about the Dead Letter Office at Washington as an afterthought or whether it was part of his original story plan for "Bartleby the Scrivener, A Tale of Wall Street." The opening paragraph, which mentions something, a rumor, that will be revealed in the "sequel" doesn't really shed light on Melville's intentions, as the statement in the opening paragraph could itself been an afterthought.
One thing is clear, however, whether as part of the original intention or as an afterthought, had Melville put the entire "sequel," as he called it, earlier in the story, the moralizing and philosophizing would have made it unnecessary to read the story (except out of curiosity) because the import of Melville's interpretation of his story is all contained right there in the sequel and reveals that Melville intended Bartleby to be the representative of those whom circumstances have cut off from the love and aid of others, even though love and aid have been sent.
In this interpretation, Bartleby symbolizes the despairing ones Melville lists in the sequel and the lawyer represents the letters that carry hope, relief, love, relief, charity, and good tidings. Just as the letters are never successful in administering their love and aid--being waylaid as they are in the Dead Letter Office--so the lawyer was unsuccessful in delivering his love and aid to Bartleby though he did try to do just that an more than one occasion.
In reflection, it seems that had Melville mention only the fact of the Dead Letter Office earlier in the story, understanding of the story would have come a lot easier, but a disadvantage of easier understanding would be that Bartleby would lose a good deal of his mystique and the reader would attend less closely. Further, the sense of mystery and suspense would be instantly dissolved rendering the story a simple reminiscent narrative instead of a suspenseful philosophical discourse. This also means that what critics call the ambiguity (uncertainty of meaning) of the story would be eliminated; for some critics, loss of ambiguity would be an improvement, while for others, it would be the loss of one of the story's greatest assets.