But, why do I care about the voltage to earth as long as I have voltage to loads?
For normal operation, it doesn't really matter. A 240V load will operate with one conductor at earth and the other at 240V above earth, or each at 120V from earth but 180 degrees out of phase, or even one at 480V and one at 240V. So why are systems grounded? For safety. Since almost all loads and wiring are in close proximity to objects at or close to earth ground potential, safety reqirements (including insulation type, air gap requirements, etc.) are determined in part by the maximum voltage to earth.
