Why does O'Brien include Norman's letter in the story?

The letter touched O’Brien, but also shook him up. The vet had congratulated himself on adjusting so well. He had transitioned straight from Vietnam to Harvard. But receiving the letter was in fact part of the process of O’Brien figuring out just how deep his own trauma cut.

O’Brien resolved to tell the story of Bowker’s perspective on Kiowa’s death. The writer tried to incorporate the story of the shit field into the novel he was working on, but to make it fit he took out the lake and most of Kiowa’s story. Bowker was disappointed with the result. O’Brien says that he was frustrated, too.