The purpose of the armed forces, in the United States, is to protect the nation's citizens and territory from threats. While this is usually defensive in nature, meaning reactionary (i.e. the Canadians attack across the border) the government also employs the military in what's known as "preemptive strikes" (attacking a nation or organization that intends to do the U.S. harm but hasn't actually done it yet.)
Historically, without having a military the United States wouldn't exist. It was an organized revolutionary army that earned us freedom from Great Britain. It was our military that helped win both World War I and World War II, so without us Europe would be a very different place. In this historical respect the value of military forces worth is proven. But what about in today's world, where we don't necessarily fear invasion?
Imagine that every country is a child that goes to school. Some wear fancy clothes while some wear rags. Some arrive in sweet cars while others hoof it. The thing they all have in common is that most arrive to school with a bully in tow. The bully's just is to make certain that child (country) isn't threatened in any way or picked on. To many people, considering this situation, having the biggest, toughest bully in the place would be an advantage. The U.S. has the biggest bully of all, the equivalent of Godzilla amongst lizards.
This has certain benefits:
- Nobody tries to attack our nation, conquer our land, and enslave our people. It sounds impossible, but consider Iraq attacking Kuwait or North Vietnam invading South Vietnam.
- Nobody inside the United States attempts to organize a revolutionary army to overthrow the government with serious ideas about successfully using it.
- Having a strong military better ensures that foreign nations honor their commitments and agreements with us.
- A military force provides protection to ships against modern-day pirates.
- Our military forces allow for the protection and/or extraction of American citizens living in foreign countries.
- The military can be used in cases of natural disaster to provide security and rescue operations.
- The military provides millions of jobs, directly or indirectly, and provides valuable training/conditioning for those who enlist.
- The military is able to act to impose America's will or morality on other nations as it deems necessary. It has the resources and training to intervene around the world, when it is ordered to do so, to conduct "missions of mercy."
Notice that what is true of the United States military is not necessarily true of other nations. In some nations the military has large law enforcement duties, may be directly involved in governing the nation, or may act to repress citizens and protect those in power. Some nations, such as Haiti, got so sick of the military being used against its citizens that it abolished the institution altogether.
All in all it would be better to live in a world where no military forces were needed, but considering the world we have the safest course of action is to have the baddest bully on the block.