Lightning is the number one reason we have rods, now a mere sibling in a family of Grounding Electrodes.
The second purpose is to provide a local connection between the neutral and ground. At the service entrance, all equipment grounding conductors are tied to the service neutral by means of the system bonding jumper. In this way, the neutral (the uninsulated wire that has the two or three insulated wires wrapped around it) is not the only way that unexpected energy can find its way to an earth ground. The transformer where the power is supplied has the center tap of its secondary tied to an earth ground, and that is also where your service neutral ends up, too.
Any questions?