In Poe's classic detective story "The Purloined Letter," an unnamed narrator is used mainly as a plot device to move the story along and to serve as a foil for Dupin and as proxy (stand in) for the reader. The narrator asks Dupin the same rhetorical questions that we might ask if we were there. In this way, he is both "in" and "out" of the story: "in" as a character who can ask questions, but "out" as someone who cannot answer them and, therefore, solve the case (like the police and the reader).
So says Enotes:
An unnamed narrator, acting on the reader's behalf, asks very direct questions—which Dupin's character would not be likely to ask—that advance the plot. It is the narrator who asks the Prefect, "And what, after all, is the matter at hand?" thereby prompting the Prefect to start his tale. When the Prefect is being vague, the narrator says, "Be a little more explicit." When Dupin says something that is even slightly confusing, the narrator repeats the confusing part. An example of this is when the narrator repeats the phrase, "Its susceptibility of being produced?" which Dupin stated to indicate that having the letter handy so that it could be destroyed, if necessary, is equally as important as having the letter at all.
We have a more unreliable narrator in Melville's "Bartleby the Scrivener." Here, the narrator--like the reader--cannot make sense of Bartleby's odd behavior, his "No in thunder." Whereas Bartleby is an outsider in society, the lawyer/narrator is an insider. Instead of a plot device who merely asks questions, like in Poe's story, the lawyer frames the story. He is like the police in Poe's story: he is coming to us the reader (Dupin), are asking us to help solve Bartleby. Bartleby is the case unto himself AND the lawyer's narration is also a case to be solved. Enotes, again:
Melville's use of an unreliable narrator is the stylistic technique most remarked upon by literary scholars who have examined "Bartleby the Scrivener." By relating the narrative from the lawyer's point of view, Melville adds a level of complexity to the story that greatly enhances the number of ways it can be interpreted. As a narrator, the lawyer is unreliable because the reader cannot always trust his interpretation of events. The lawyer, as he himself admits, is a man of ''assumptions,'' and his prejudices often prevent him from offering an accurate view of the situation. This becomes clear early in the story when the lawyer's description of Turkey's unpredictable behavior in the afternoons begs the obvious conclusion that he drinks during his lunch hour. Yet the lawyer is evasive about the matter, perhaps intentionally so. Thus, when the lawyer interprets Bartleby's behavior, the reader must decide carefully whether or not the lawyer is accurately perceiving events.