The point of the material plane (in D&D) is to encompass all of the inner planes, fire, water, earth, air, positive, and negative. It is the mix of these planes. The elemental planes take one chunk out of the material plane, and make it pretty much dominant (hence why fire spells are boosted in the plane of fire, and water is weakened). The mix of these elemental planes are what makes up nature, and life and death (positive and negative)