Ground loop.

In Australia we have a connection at the meter box that is the MEN. The Main Earth Neutral Link. This is the only place on a house that the Neutral is connected to the Earth.
If the (AC) neutral is connected to a DC ground, earth of a machine in Australia the Safety switch (Earth leakage breaker, Residual current detector are other names) would switch the power off.
This leads us to use dual pole switching often to isolate the AC line and Neutral. The Earth wire is used to ground the machine.
This has advantages in isolation and allows our machines to have a single earth point to as ken says eliminate earth, ground loops. These are currents that flow in the earth wires if they are joind in loops. (Like a single turn of wire added to the transformer above.)
This makes our eacth wires go out from a single point like a star.