The UN is not a policeman of the world. Members still retain full sovereignty. It is far more about agreeing standards of social behaviour and highlighting where they are not upheld. The strength of the UN lies in agreement about international issues such as asylum rules and marine laws. It's original purpose was to define a post war western/global understanding about how to prevent what had happened and to set a high standard of communal behaviour. What they cannot do is insist on it. That is a sovereign issue. It can also point the way to where international investment can go by highlighting what is going on there. Today the most active parts include a peace keeping force, huge support for education and health in poor areas and as a definition of human rights against which judgements can be made. It is what is known as a soft power, but its early conventions stand as a model for national values, including the USA. They cannot prevent revolutions but can influence law such that they are far more difficult to accomplish. IMO it's use is measured by what kind of a world would it be if it weren't there.