We needed a united world, the UN is important in not only uniting the world but in helping the world work through the major problems we face as a collective.
America is the biggest economy in the world. We need America in the United Nations to help this process of world unity, world peace and collaboration in solving world problems.
The United States cannot take action against other countries that committed genocide because the security council would just veto on that. In reality, the UN is actually helpless in protecting the free world, rather it has become a mechanism in which countries like Russia and China would use to prevent any American intervention. Whenever there is crime against humanity committed, the UN turns blind; only the US wants to take action, but is prevented by the organization of the oppressors (United nations).