Final answer:
The flashback in the story serves to explain why Peyton Farquhar is being hanged.
Step-by-step explanation:
The main function of the flashback in the story is to explain why Peyton Farquhar is being hanged. The flashback provides background information on Farquhar's character, motivations, and actions leading up to his eventual capture and sentence. By going back in time, the story helps the reader understand the reasons behind Farquhar's impending execution.