From the perspective of United States interests, at best it serves a very limited useful purpose. And the UN does nothing important without the backing of the U.S. military anyway. The United Nation's main current functions are to: 1. Provide a forum for anti-Semitism thereby allowing numerous oppressive governments and despotic regimes to deflect criticism from themselves by demonizing Israelis. 2. Occasionally acts as a source of legitimacy for U.S. international military intervention. It's become more trouble than it's worth. I'm not completely convinced that the United States should withdraw, however, I see no justification for our continuing to provide headquarters and facilities on some of this country's most prime real estate.

It serves a noble, useful, and even necessary purpose. That being said, it has become ineffective in many ways and needs a huge overhaul.