Syria, anybody remember that place? Its conflict continuing is proof that the United Nations doesn't work, and it's proof that they need to step in when things reach a certain point. Genocide for month after month after month, for example. The United Nations needs to stand for something and step in when it needs to, not just when its convenient.
The United Nations should get involved in conflicts that are internal/limited to a specific country. There are atrocities happening around the world, and they may not stop unless people stand up and get involved, as a unified group. A country is unlikely to change its ways on its own. One example could be North Korea. This country is notoriously violating human rights, yet it will never stop because it has blocked itself off from the rest of the world. How can change come about if people refuse to get involved?