RoyC's answer explains why there is a ground rod. I would add that the ground rods are distributed throughout the system. Across the overall utility system, the ground rods collectively have a low impedance. However, the ground rods on your local transformer loop have a high impedence (25 ohms if you have one, and generally higher if you require more than one). So your individual ground rods don't do much, but collectively your neighbors' ground rods (when correctly wired) are keeping your ground conductors at earth potential.
This insight enables us to answer this part of your question:
This question is not specific to a fault condition. If the neutral and
ground are tied together at the panel what prevents current from the
neutral wire flowing to earth through the ground rod?
Your local transformer loop has a highly conductive utility neutral, so the voltages on that neutral are small. Therefore the currents through your relatively resistive local ground rods are small.
Of course this argument breaks down if somewhere there is a rogue connection from 120 volts directly to earth (typically because a utility neutral opened up somewhere, or just some kids trying to fry underground bugs). In that case, the rogue connection is fighting against all the good ground rods in the system. The ground rods then form a voltage divider with the rogue earthing, leaving your ground conductor voltage at 120 volts times the ratio of the rogue impedance to the collective impedance of all the good rods. So the more properly connected ground rods, the better.
Bottom line: the ground rods are a community service: you help your neighbors and they help you.
