It created the framework of international law for the codifying of "crimes against humanity". It was later used in the Nuremberg Charter (article 6c).
It is easy to explain why the Universal Declaration is no more effective than the 1915 Declaration. Just as Turkey was not punished for its crimes - because the victorious Allies were too busy competing among themselves for the spoils of the defunct Ottoman Empire to care about the genocide of the Armenians - neither will the Universal Declaration be effective as long as nations pick and choose whom they will "punish".
When political expediency comes against moral rectitude, no one will get a prize for guessing which will win. The big and the strong will only pick on the small and the weak. And since it is only the big and the strong who can do anything about violations of human rights, only the small and the weak will be punished.
Pious words and intentions never deterred a killer.