This article posted in a comment above explains it well: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html Essentially, the ground ro

This article posted in a comment above explains it well: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html Essentially, the ground rod acts as an "anchor" holding the neutral wire and the ground at the same electric potential. If there was no ground rod, the earth and the circuit would be "floating" relative to one another, and a dangerously large voltage could develop between them.