The main function of the World Trade Organization (WTO) is to promote trade between the various countries of the world. Mainstream economic thought holds that free trade between countries is good for all countries that participate. For this reason, the WTO was created to try to increase the amount of trade happening between as many countries as possible.
What the WTO mainly tries to do is to create and maintain rules that promote free trade. The organization starts by trying to help various countries come to agreements on the subject of free trade. The WTO cannot force countries to join. It can only help them hammer out treaties in which they agree to a common set of rules. Once these treaties are in place, though, the WTO can work to ensure that the countries adhere to the rules set out in those treaties.
Because the WTO thinks that trade is good for all countries, it also tries to help less developed countries build their ability to trade. It sends experts to countries to help them build this capacity. In doing so, the WTO hopes to include more countries in the global trade network.
Thus, the main functions of the WTO are to help countries reach agreements on trade, to help enforce those agreements, and to help build the capacity for trade. All of this is meant to bring on economic development.