The School of the Americas, based in Fort Benning, Georgia was founded on United States' landholdings in Panama in 1946. At the time, the United States governed the territory surrounding the Panama Canal and used this land for many purposes - including the School of the Americas, a training camp for South and Latin American geurilla fighters and dictators' officers.
In 1984 it was relocated to its present location due to the Panama Canal Treaty, a tenet of which states that all U.S. landholdings in Panama would be forfeited at this time. Today, it is called the Western Hemisphere Institute for Security Cooperation though its function remains the same.
"Over its 56 years, the SOA has trained over 60,000 Latin American soldiers in counterinsurgency techniques, sniper training, commando and psychological warfare, military intelligence and interrogation tactics. These graduates have consistently used their skills to wage a war against their own people. Among those targeted by SOA graduates are educators, union organizers, religious workers, student leaders, and others who work for the rights of the poor. Hundreds of thousands of Latin Americans have been tortured, raped, assassinated, 'disappeared', massacred, and forced into refugee by those trained at the School of Assassins". (http://www.soaw.org)
There are many reasons that the U.S. would choose to operate such an institution. Firstly, it keeps South and Latin America relatively unstable. This is in U.S. interest because major corporations based in the U.S. use large amounts of cheaper foreign labor in South and Latin America to cut production costs. In the same vein, an unstable South and Latin America is one indebted to the U.S. or, more abstractly, to the International Monetary Fund (IMF) which the U.S. has a large stake in. This indebtedness keeps these countries undeveloped and rich in cheap labor for American corporations.
Secondly, an unstable South and Latin America is one that can not challenge the United States' power. If Mexico or Columbia or Peru became a formidable military or economic force, it would create a competitor to the United States in close proximity.
Thirdly, the United States likes a say in foreign governments - especially those close to it. By training resistance fighters and releasing them into a country, the United States can effectively topple a government they have a problem with and instate a new one that agrees with them more.
There are a multitude of other reasons the United States has for the School of the Americas, but these are some of the most important ones. Essentially, its purpose is to help the United States maintain its superpower status, leeching off of weaker nations and keeping them in a state of poverty so that they rely on the United States, thus ensuring their loyalty.