Maybe it's tied together somehow with the devil fruits. Think about it. We haven't seen a tree or anything that produces these fruits. Where do they come from? How do people obtain it? It's a silly idea but maybe Raftel is where the DFs come from and the fruits just float from Raftel.
“Pirates are evil? The Marines are righteous? These terms have always changed throughout the course of history! Kids who have never seen peace and kids who have never seen war have different values! Those who stand at the top determine what's wrong and what's right! This very place is neutral ground! Justice will prevail, you say? But of course it will! Whoever wins this war becomes justice!”
― Donquixote Doflamingo