Answer:
To spread Communism
Step-by-step explanation:
The Cubans were communists, and the main goal of the Soviet Union was to spread communism throughout the world, and although they made the policy of Detente (peaceful coexistence with the US and its allies), their main goal was to replace Communism with Capitalism.
However, there was also another goal. The US had stationed nukes in Turkey, and the Soviets were feeling really threatened. As a result, they wanted a Communist country that they could put nukes in.