Lord Zack's Planescape
Each of the elemental planes are dominated by a single element. They seem to be the source of that element and the ultimate example of it. It is unclear why the elemental planes exist. Some posit that the the primordial gods created the elemental planes as a testing ground for the material plane. Other believe that they existed independently and were then used to create the material plane. Yet others say they came into existence as a result of the creation of the material plane.