At the very end of The Rules of Survival, Matthew reveals that though the letter addresses Emmy, he's actually been writing it to himself this whole time. She'll never even read it:
So. Emmy. Little sister. You're never going to read this, are you? I'm never going to give it to you. I didn't write it for you. I wrote it for me.
I wrote it to work my way through the story of what formed me. (E.1-2)
But does this mean that telling this long, painful story was all for nothing? Not at all. Matthew knows that it has been valuable to write down this whole story because doing so has helped him work through all of his complicated, raw feelings, to understand what's happened to him and, in doing so, to be able to move past it.
And that's priceless.