The UN facilitates diplomacy, instead of NATO deciding to do something about russia, they have a security council vote and with diplomacy form blocs of nations on each side of a conflict. Before the UN,an alliance like NATO will do something about russia and just drag in others. Not just the great war, but WW2 is a good example, diplomacy with italy and japan would have prevent or deescalate many of their aggressions, but at the same time response against the axist could have been global, imagine not waiting for an axis invasion but sending troops as part of a coalition force before they advance to you, problem was there was no place where nations could agree on a response.

If China starts invading asia for example, the UN can be used to sanction or even coordinate a resistance effort both diplomatic and kinetic before it gets out of control.