The nukes sent to Cuba through Operation Anadyr were because

1. The Soviets were responding to nuclear Jupiter missiles the US placed in Turkey (right on the USSR’s doorstep, and which Kruschev managed to negotiate a withdrawal of alongside the nukes sent to Cuba as a deal on the condition that this withdrawal remain secret so Kennedy could protect his image)

2. As a response to West Germany violating the agreed upon borders and placing a NATO base in East German territory, and

3. To prevent a full-scale invasion of Cuba that the US was preparing for.