This is an uninformed understanding of the United Nations. All nations deliberate as equals in the General Assembly. The Security Council has permanent members and a rotation of other members, but only five have veto. This means everyone can pass a resolution, but it can't be enacted if it is vetoed by one of the five permanent members. There are 15 members of the Security Council. This includes five veto-wielding permanent members- China, France, Russia, the United Kingdom, and the United States- based on the great powers that were the victors of World War II. There are also 10 non-permanent members, with five elected each year to serve two-year terms. This basic structure is set out in Chapter V of the UN Charter. The current non-permanent members are Argentina, Australia, Azerbaijan, Guatemala, Luxembourg, Morocco, Pakistan, Rwanda, South Korea, and Togo. It may not be perfect, but it works.
To pay dues to keep that huge building running. They can also go to the UN if their country is ever in need of help, such as AID or DISPUTE RESOLUTION with another member country. It is a good way to network and to know what is going on in the world.