To deter future wars. After the end of World War II, the US and the Soviet Union emerged as the two superpowers, and they began a nuclear arms race. Each side developed and stockpiled large numbers of nuclear weapons, which they believed would deter the other side from attacking them. This strategy was known as mutually assured destruction (MAD).
To maintain political power. Nuclear weapons have also been used to maintain political power. For example, the US used its nuclear weapons to pressure the Soviet Union during the Cold War, and the Soviet Union used its nuclear weapons to pressure the US and its allies. Nuclear weapons have also been used to intimidate smaller countries, such as Iraq and North Korea.
To spread influence. Nuclear weapons have also been used to spread influence and gain prestige. For example, the US used its nuclear weapons to become the world's leading superpower after World War II, and the Soviet Union used its nuclear weapons to become a major world power.
To gain technological advantage. Nuclear weapons have also been used to gain technological advantage. For example, the US used its nuclear weapons to develop the atomic bomb and the hydrogen bomb, and the Soviet Union used its nuclear weapons to develop the intercontinental ballistic missile (ICBM).
To protect national security. Nuclear weapons have also been used to protect national security. For example, the UK used its nuclear weapons to deter a Soviet invasion during the Cold War, and India and Pakistan have used their nuclear weapons to deter each other from war.
To achieve geopolitical goals. Nuclear weapons have also been used to achieve geopolitical goals. For example, the US used its nuclear weapons to force China to agree to the Korean War Armistice Agreement, and the Soviet Union used its nuclear weapons to force the US to withdraw from the Cuban Missile Crisis.