Near the end of 12 Years a Slave, Solomon describes how months go by after Bass sends out letters detailing where Solomon is being unjustly enslaved. When a carriage finally arrives to escort Solomon home, he pauses the scene and includes a flashback to explain why it took so long for his friends and family to respond to the letters, introducing it as so:
Having now brought down this narrative to the last hour I was to spend on Bayou Bœuf—having gotten through my last cotton picking, and about to bid Master Epps farewell—I must beg the reader to go back with me to the month of August; to follow Bass’ letter on its long journey to Saratoga; to learn the effect it produced—and that, while I was repining and despairing in the slave hut of Edwin Epps, through the friendship of Bass and the goodness of Providence, all things were working together for my deliverance.
The flashback creates narrative tension (readers want to know who steps out of the carriage and how Epps will respond) and also shows how hard it was, during the years when slavery existed in the South but not in the North, to do something as simple as freeing a man who was already born free.
In the flashback chapter, Solomon walks readers through every single step that Henry B. Northup had to take to free him, including: getting the governor of New York involved, going to Washington, D.C. to receive support from a senator, and recruiting a sympathetic lawyer in Louisiana to help in locating Solomon and freeing him in accordance with local legislation.
Solomon likely also included the flashback in order to assure readers that his family and friends in the North had not abandoned him. It was entirely due to an unjust “justice” system and the violence of slavery that they took so long to locate him and bring him home.