If you see in WW II a lot of places were conquered by Hitler just because their army was not prepared. And in todays world there are constant threats from terrorists and also from the nations who are preparing or HAVE nuclear weapons. If USA did not have the army and was not prepared for a war, former USSR would have attacked it during the Cold War. The main reason there was not a real war between them was due to the fact that both the nations would be completely destroyed, had there been a war between them. The only reason that might prevent WW III is due to the fear of world destruction due to the massive army and their equipments. Hence, as said by my Big B, it is a sort of preparation for anything in the future and also a sort of prevention of any massive wars in the future!