Final answer:
The flashback's main purpose is to create suspense and tension, while also contributing to the story's exposition and aiding in the development of the theme which emphasizes the significance of trusting one's own knowledge and training over fear.
Step-by-step explanation:
The main purpose of the flashback in question, which includes dialogue that suggests that Mrs. Darrell's warnings about Gasbang and Hughes were probably incorrect and foreshadows issues with the claim, is to create suspense and tension in the narrative. This technique is often used by authors to build interest in the story's conflict, and in this case, it sets up a major conflict. Additionally, the dialogue is a tool through which the author can develop the theme of the narrative, demonstrating the importance of relying on one's own knowledge and training instead of succumbing to doubt and fear.
Using dialogue to reveal the backstory, the author provides further context to the sequence of events in the story, contributing to the exposition and engaging the reader. Such flashbacks coupled with dialogue allow the readers to witness the story unfold in a more vivid manner, creating a realistic recreation of events that maintains their attention and elicits emotional investment.