on Jane Friedman site:
Flashbacks are scenes that take place prior to the narrative arc of a story. They can illuminate any number of story elements, from revealing the origins of an unusual habit to new information about a relationship. Flashbacks can give the reader a depth of context not available in the primary narrative.
Alternately, flashbacks can help the reader understand your reaction to an event in the primary timeline. For example, maybe you had a fight with your spouse, and the exchange reminded you of how you used to cower in your closet when your parents fought. While you can tell with that line, showing via a flashback can be more engaging for the reader.
However, flashbacks can be tricky to write. Written unskillfully, flashbacks can leave a reader disoriented and disengaged.
What follows are the five mistakes I see most often in memoir manuscripts, though these principles are also relevant to fiction. If you’re writing fiction, just substitute “your main character” for “you.”