What other purpose does it serve? None. But because the neutral is grounded at the transformer, ground becomes a return to the transformer. Without that connection and the ECG connection at the panel, ground would not provide a return. In that case if you touched a hot wire to ground nothing would happen. If you had a hot shorted to the case, of say a washer, and you touched the case and a water pipe you would not get shocked. The only purpose of the ECG is lightening protection, and bonding is a compensation for a problem the lightening protection causes. A GFCI dose a much better job because it dose not require 15 or 20 amps to react.