Why did the poet in dulce et decorum est says he saw the scene in all his dream?

The poet in "Dulce et Decorum Est" describes seeing the scene in a dream to highlight the lasting impact of the traumatic events he experienced. The use of the dream motif emphasizes the haunting nature of the war experience and the persistent memories that continue to affect the speaker even in his subconscious. It serves to convey the emotional and psychological toll of war on the soldiers.