It shows how Rachel grows as a character overcomes conflicts. It tells us her inner thoughts and how she feels throughout the story. The 1st person writing also helps us relate to Rachel about difficult and embarrassing moments and about emotions.
I think the author made the story in 1st person to show us how Rachel evolves a character and overcomes challenges. Also we know Rachel is shy and doesn't speak up for her herself it shows her inner conflict and how she gradually tries to overcome it.
