Quite simply, to retaliate against a possible USSR attack on Europe and/or America.
After WW2, European nations grew increasingly speculative of the USSR, which stood alongside United States as the sole power. As a bastion for communism, the USSR takes dangerously risky moves, such as the closure of Berlin (which leads to the Berlin Airlift). The USSR also developed nuclear weaponry, which gave West Europe enough to worry about their power-hungry Eastern ‘foe’.
NATO was formed in 1949 to combat a possible USSR attack on Europe and/or America. An American idea at first, NATO pools several nations in Europe and the Americas, all fearful of the growing USSR threat.
Of course, the USSR would form a Warsaw Pact to respond to NATO. The creation of NATO shows the growing complexity of the Cold War, and its services continue to stand today.