Their goal is not to enforce laws. It's to provide a regular space for nations to talk. It's diplomacy, which some see --not without reason-- as a particular kind of hypocrisy, but in a world-wide context.
Under the UN, there are organizations that do international work, governed by an international alliance. These do work that otherwise would be fragmented. That's not impotent.
I dont mind the UN building. But as an IR Realist, I find it a bit insulting that the UN pretends it has power. It only is used as moral coating to protect the status quo.