Wait what? Geography teaches you demography?
Oh and @phantomgoth sorry for the derailing. Anyways to follow up and finish the main idea, basically the first qorld countries also happen to be supporting third world ones with not only releif efforts but economic interactivity.
Without first world countries society would most likely regress to the state at which third world countries would be in, which would be worse off at that point than before. And then I assume no one would be left to operate nukes, so assuming they were never dropped then the threat of nukes would disappear and the fragile peace holding the entire world in a ceasefire would evaporate due to the idea that no longer can your country have chunks of it instantly die.