Why is the Earth a useful reference of electric potential?
You're taking this "earth" or "ground" thing too literally.

If I tell you: "There's 100 V DC on this node." Then then you would assume that I mean "100 V DC relative potential to the ground node of the device I'm measuring at that moment.

You'd call me crazy if I said: "There's 100 V DC on this node relative to the metal hull of an oil-tanker which is in the middle of the Atlantic Ocean".

It would be inconvenient to actually measure like that. There's no guarantee that the oil tanker's hull is properly grounded (also: grounded to what ? The bottom of the sea ?)

This silly example demonstrates that "ground" or "earth" is just a local reference point. We define it as having a zero potential voltage. You always need 2 nodes to measure a voltage, a voltage is actually a potential difference. It is extremely convenient to make one of those potentials zero. And we call that one "ground" or "earth".