In Eliza's opening letters to her friend Lucy, she recounts her previous engagement to Mr. Haly, the man her parents initially chose for her to marry, in a flashback:
As their choice; as a good man, and a faithful friend, I esteemed him. But no one acquainted with the disparity of our tempers and dispositions, our views and designs, can suppose my heart much engaged in the alliance.
By the time Eliza actually writes this passage, Mr. Haly has been dead for several months, and she is living with General and Mrs. Richman in New Haven. The flashback allows the reader to gain an understanding of Eliza's history and the way her unwanted engagement has shaped her current desire to enjoy her social life without thinking about the future. Because of this flashback, the novel can capture Eliza's extensive backstory while taking place over just a few months in her life.
Notably, given the fact that Lucy and Eliza are close friends and keep up a steady correspondence, Lucy would likely already know about the circumstances of Mr. Haly's death and Eliza's feelings about it. While essential for the development of the novel's plot, the flashback reads as somewhat artificial authorly intrusion into the narrative. In this sense, the flashback illuminates the tension between creating a realistic representation of a real person's correspondence and making that correspondence legible to readers which lies at the heart of many epistolary novels.