The answer to the question of “why first use” relies heavily on Cold War-era thinking and the doctrine of deterrence theory, which assumes you can scare adversaries about the consequences of attacking so much that they won’t ever actually do it. During the Cold War, nuclear war planners firmly believed that a U.S. threat of first use deterred even a non-nuclear invasion of Europe by the Soviet Union. Even today, governments of European allies like the security of a first-use policy. Similarly, Asian allies like Japan and South Korea rely on the threat of U.S. first use to deter any form of aggression from their neighbors. The U.S. also uses this threat to deter significant, non-nuclear, strategic attacks. Exactly what that means is purposely ambiguous, but it would have to be an extreme circumstance and could include attacks on civilian populations or nuclear forces.