how did Erik Vandenburg save Hans Hubermann's life in the first world war?

In WW1 Erik saves Hans's life. The day the platoon is to go into battle, the Sergeant asks which one of them has good handwriting. None volunteered. The Sergeant says that whichever man does will not be going into battle, yet no man wants to seem like a coward. Erik nominates Hans, who is sent to write letters for the captain. The rest of the men are all killed.