The letters

Near the beginning of each story there's a letter that a character is seen re-reading. In "Franny," it's the letter from Franny to Lane, which he takes out of his pocket and reads again while he's waiting for her train. In "Zooey," Zooey is in the bathtub re-reading (for the umpteenth time) the letter from Buddy. What do you make of this parallel structure?