Any time you have a writer writing a story about a writer, you just know that literature is going to be a big deal. "For Esmé" is no different. While this story is, on one hand, about a soldier going through a war, it's also about a writer learning how to find his voice. There are several moments in the story where we are reminded that our protagonist is a writer, both by his words and his actions. So, we constantly have to ask ourselves what the events of the story have to do with his ultimate destiny – which is to become the guy writing the story (whew, is that meta or what!).
The ultimate destiny of the narrator/Sergeant X, to become a real writer, could not come to pass without his experience in the war.
One of the "faculties" that Sergeant X loses and regains after the war is the faculty of writing; we see him lose control of his own ability to write in Gaufurt, but, from the story's frame narrative, we know that he ultimately regains it.