Physical death is the separation of the body from the soul. This creates a problem, because without bodies we are incomplete. Resurrection, which reunites body and soul, is the solution. It is not merely the bringing back to life of a corpse, but rather the re-creation of the body to make it fit for eternity. The answer is that a body is part of our basic nature.
Bodies are basic to our nature.
The reason we are decaying and dying is not because we have bodies. It is because sin has infected our bodies. God designed us as physical and spiritual creatures who were to find fulfillment through physical and spiritual activities. Just as having a physical nature without a spiritual nature would make man an animal, so possessing only a spiritual dimension without a physical shell would make him more like an angel than a human being. New bodies are also necessary to fulfill God's perfect justice.