After Dick finds his bank book missing and begins to suspect that his boardinghouse neighbor, Jim Travis, stole it, Alger uses a flashback to show the crime from the perspective of Travis, who is indeed the thief. Not only does the flashback state exactly when and how Travis steals the bank book—it also tells the reader a lot about him as a character:
Travis, like a very large number of young men of his class, was able to dispose of a larger amount of money than he was able to earn. Moreover, he had no great fancy for work at all, and would have been glad to find some other way of obtaining money enough to pay his expenses.
This flashback is immediately notable because Alger rarely strays from Dick's perspective or shows events that happen outside his presence. The sudden shift to Travis's perspective alerts the reader that this is an important moment in the novel's progress. In this sense, the flashback creates suspense and intrigue because the reader learns what has happened before Dick does and must wait to see how the protagonist handles the theft.
The flashback also plays into Alger's thematic concerns around theft, industry, and personal responsibility. Previously, Alger introduced Travis as a "coarse-looking fellow" and alluded to his bad character. Now, he uses the flashback to outline Jim's motivations, making sure the reader knows that Travis has "no great fancy for work" and consistently spends more money than he can earn. Like Dick at the beginning of the novel, Travis is faced with the problem of lacking money to live the life he wants. But while Dick aspires to a life of middle-class respectability and works towards it through honorable means, Travis simply desires unearned luxury and is willing to steal to achieve it. To Alger, this isn't an isolated case: by saying that Travis behaves like "a large number of young men of his class," the author shows that he's not writing about one person's character but a broader social problem. Ultimately, the flashback helps Alger distinguish between the hard-working and the lazy and to dispense moral judgment on those who choose Travis's path.