How does the author's use of exposition affect this story?

It builds the tension that occurs in the story: the mystery behind the letters Bella finds. It provides a resolution to the events that happen in the story: the identity of the letter writer. It is the turning point in the story, when Bella finds the message asking for her to meet the letter writer in the park. It introduces the main characters, Bella and her family, and sets the scene for the action that follows.