UN: What would a world without the Organisation of the United Nations look like?

When we see major conflicts occurring around the world and we see the Organisation of the United Nations being completely ignored, what role if any can the Organisation of the United Nations actually play? Israel recently refused to provide visas for UN officials to enter Israel and Israel is a member of the UN. What does this show? 

When major players use the UN when it suits them and ignore the UN the rest of the time, what is then the point of having an Organisation of the United Nations?

The UN was supposed to be a forum to prevent conflicts when in fact UN is a forum that cannot prevent conflicts and that is reduced to witness conflicts having little power to do anything about them.

The big powers do as they please and often completely ignoring the UN. Bombing in Yugoslavia took place without a UN mandate. The second invasion of Irak and the invasion of Afghanistan took place without a UN mandate. With regards to what is happening in Palestine, the UN was completely bypassed and even decisions made regarding assistance to civilians were totally ignored.