I think you have to approach this question under the right understanding of what the UN is actually supposed to do. Generally, it was set up to provide a forum for international cooperation. Some successes include Sierra Leone and Burundi, but mind you there are more failures in Africa than successes. Nonetheless, the UN and other humanitarian organizations provide much needed medical, residential, and resource support to developing countries.
Many people think the UN fails at international cooperation. This might be true, but this due, in part, to sovereignty. Nothing discussed in the UN is legally binding since a leader keeps sovereignty over their nation. On the other hand, countries have used the UN for everything from putting sanctions on Russia (which, whatever your opinion, has been partly a success) to environmental reform (UN climate summit is happening now!), and even scientific research.
All in all, the UN does a lot more than people think. So I wouldn't say it's useless by any means.