Grounding system has three main purposes:

Overvoltage protection

Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system wires. Grounding provides an alternative path around the electrical system of your home or workplace minimizes damage from such occurances.

Voltage stabilization

There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings of electrical deistirution systems as a nearly universal standard for all electric systems.

Current path in order to facilitate the operation of overcurrent devices

This purpose of grounding is the most important one to understand. Grounding system provides certain level of safety to humans and property in case of equipment damages.