Despite its tragedies, war allows a country to gain new lands or to protect its lands or interests from would-be invaders. Also, war and the threat of war contribute to major economic sectors, including technology and manufacturing. Companies that provide equipment, supplies and services used by the military would lose their revenue streams if there were no wars.
Military aggressors typically pursue land or territorial control, or they want to proactively dissuade rivals from coming after them. Throughout time, the winners of wars and battles typically end up with new land to develop. More land ownership means increased resources for a country's population and more space for economic development.
Billions of dollars are spent annually on national defense by the United States. Much of that money goes to builders of tanks, guns, helicopters and other weapons and vehicles. Military logistics service providers also serve government customers. Inventors look to develop new products that enhance the ability of soldiers to do their jobs. Perpetuation of wars and global enemies is vital to sustaining these enterprises. In addition to supporting major business sectors, the military provides careers and income to many Americans.
An indirect advantage of war is that it contributes to patriotism. Americans have patriotic pride that started with the fight for independence and continued through other successful wars that have followed.