Question

why was the united nation made for and what is it's use because they do not promote peace between nations

Answer 1

Well, they try.

The UN has 4 main purposes:

To keep peace throughout the world;
To develop friendly relations among nations;
To help nations work together to improve the lives of poor people, to conquer hunger, disease and illiteracy, and to encourage respect for each other’s rights and freedoms;
To be a centre for harmonizing the actions of nations to achieve these goals.
http://www.un.org/en/aboutun/index.shtml