The four spheres for the elements provided the explanation for what we now call gravity. Like attracts like: thus earth falls through fire, air and water; air, on the other hand, rises through water, and water rises from earth in springs. A sphere for fire (though invisible) was needed to explain the fact that fire appears to rise through air.

Everything under the sphere of the moon was subject to decay and change; above was immutable. Of course, this hypothesis led to some beliefs that seem odd to us -- that comets were closer to the earth than the moon, for example, since they could clearly not be in the unchanging sphere that contained the "fixed" stars.

More on the four elements.