What is the purpose of "In the Field"?
One way of approaching such a question is to think about the title of the story and how it relates to the story itself. In this story, which discusses the death of a soldier, Kiowa, and in particular how it happened and why, the quote relates to how things "in the field" are so different compared to how things are from the standpoint of those who are not in the thick of the fighting. When thinking of Kiowa's death, Jimmy Cross understands that "there had to be blame," and that you could blame a huge number of different people or things, from the enemy to "an old man in Omaha who forgot to vote." However, note how the text goes on to compare such quibbling with being actually involved in the fighting itself:
In the field, though, causes were immediate. A moment of carelessness or bad judgement or plain stupidity carried consequences that lasted forever.
Whatever arguments or blame can be apportioned outside of the arena of war, blame is something that is very different for the soldiers who are involved in the war themselves as mistakes have immediate and often catastrophic impact. The purpose of this story is therefore to establish the massive difference between those "in the field" and those who are not.