The Department of Defense has some 400 people involved with maintaining good relations with Congress. The legislators not only approve the Department of Defense budget, but also approve officer promotions (especially general officers) and can, if they wish, get very involved in how the military is organized and run. The Department of Defense is basically playing defense when it comes to dealing with Congress, where the huge (over $300 billion these days) defense budget is considered politically useful. A member of Congress who can bring more Department of Defense money to their voters has a better chance of getting reelected.