The Form Runes can be understood as the basic molds that were used by the greatest gods to populate the many niches of the physical plane. A god is often described as being of a particular Power or Element, and the Form determines how this force is expressed.
Everything has a Form Rune but they are rarely a source of magic powers other than the form itself. For example, all of mankind has the Man Rune but it simply means that they are men (and not gods, discorporate beings, animals, or plants).