earth as a voltage reference
With a few exceptions, we don't intentionally use Earth as a return. Power systems are still kept near Earth potential for reasons of safety.

Consider: what would happen if the power distribution lines were in the neighborhood of 10,000V above Earth potential? While there still is a normal voltage between each of the lines, and an appliance connected only to these lines would function fine, what happens if you set the appliance on the ground? What happens if a person is holding the appliance, and simultaneously holding a cold water pipe?

The only thing protecting this person from a 10,000 VDC shock is some insulation which is hopefully intact. And of course, designing for a breakdown voltage > 10,000 V is rather difficult...

It's much simpler if you can take anything which might come in contact with a person and connect it directly to the Earth. Then you know if there's any fault which might expose a person to dangerous voltages, instead of shocking the user, the current will flow through this ground system and blow a circuit breaker.