It would stand to reason that if the UN had no form of enforcing its policies, it would be a defunct political group with no real influence. The United Nations has made some mistakes, but it has also stabilized the World. It is still a very relevant venue for World leaders to make decisions that affect a global economy. The UN handles crimes committed against the people of entire nations –
without an active force many of those small nations would grow into powerful dictators with no regard for human life.
The United Nations doesn't need a standing army, lest it become known as an enforcement agency rather than an organization trying to help bring about a peaceful world. Individual nations can supply the UN with military personnel as needed, but a standing army isn't in line with the UN's mandate of resolving the world's issues peacefully.