O'Brien's descriptions of what the soldiers carry in Vietnam are part of his characterization.  What a soldier carries represents a unique part of his personality and also shows a part of who the soldier is in "The World"--life outside of the war.

His alternating between the descriptions and Cross's thoughts is a style he follows in several of his stories in this short story collection.  See "In the Field" for another example.  His purpose for using this style is to contrast the harsh reality and randomness of war and a soldier's life and Cross's desire to be back home where life is "normal."  His thoughts are an escape from the heavy responsibility of leading his men and ensuring that they and the things they carry get home safely.