This would be a good place to start if you're curious about the mission statement of the UN. This website has a lot of good information about their areas of focus. https://www.un.org/en/our-work

The work of the UN focuses on five major areas. 1.Maintain International Peace and Security. 2.Protect Human Rights. 3.Deliver Humanitarian Aid. 3.Support Sustainable Development and Climate Action. 3.Uphold International Law

They have a lot of information on their website about all of these areas. I would highly suggest going and looking at information on their peacekeeping and how they've helped in that area. They are certainly far from perfect but that doesn't make them pointless. They do a lot of good work and are making a net positive impact.