What is the importance of the wooden box Graham presents to Quinn in the novel, All Your Perfects?

When Graham presents Quinn with this box, he tells her they will open it and read their letters to one another on their twenty-fifth anniversary. He added that it would serve as a first aid kit if either of them decided they needed to end the marriage early. He believed reading about the feelings they had for one another at the beginning of the marriage would rejuvenate their love for each other.