The fact that the lieutenant can now "see many things which as a participant in the fight were unknown to him" helps emphasize that his injury has changed his role in the war and his perspective on the world.
As a participant in the fight, the lieutenant was focused on the immediate task at hand: dividing up coffee rations and preparing for battle. He was surrounded by the noise and chaos of war, and he was likely caught up in the adrenaline of the moment. As a result, he may not have had the opportunity to take a step back and see the bigger picture.
However, after he is wounded, the lieutenant is forced to take a more detached view of the war. He is no longer able to participate in the fighting, and he is forced to observe from a distance. This gives him the opportunity to see things that he would not have been able to see before. He sees the generals and aides riding into battle, and he sees the green woods that "veiled his problems." These images give him a new perspective on the war, and they help him to understand the larger stakes involved.
The lieutenant's change in perspective is also reflected in his reaction to the wounded soldier who offers him a shoulder to lean on. The lieutenant "waved him away mournfully." This shows that he is no longer able to see the wounded soldier as just another soldier. He sees the wounded soldier as a fellow human being who has been injured, and he feels a sense of compassion for him.
The lieutenant's change in perspective is a powerful reminder of the human cost of war. It shows how war can change people's lives in profound ways, and it can make them see the world in a new light.