The purpose of any country's standing military forces is to defend that country from real or potential threats, both internal and external. An army is specifically intended for land-based warfare.
Of the more than 200 nations currently in existence around the world, relatively few maintain a standing military force of any substance. Even the United States constitution limits the duration for appropriations of funds for a standing army. However, this has simply led to the U.S. military returning for new appropriations of funds on a regular two-year cycle. There are a great many concerns that revolve around a standing army, not the least of which is the potential risk of a military coup. These military led revolts against their parent government happen from time to time, so the threat is real to many leaders. However, an army is only one branch of the modern military machine. In addition to land warfare, today's military must also be able to address threats by both sea and air. This directly contributed to the creation of the world's naval and air combat forces. While the United States has one of the largest standing forces in the world, others maintain similarly organized and regular armies as well.