The ground pin, when used with properly wired electrical outlets, creates an extra level of safety for the user.
When an appliance with a connector that includes a ground pin is plugged into a properly grounded outlet, electricity flowing to the appliance has a complete path back to earth ground, giving a large additional safety margin for whomever is using the electrical device.
If the cable in the appliance is damaged, then the live “hot” conductor can deliver power directly to the body or handle of the appliance. When there is no ground path back through the cable, extension cord and outlet, then the most direct path to ground becomes through whomever is using the device. This can create a potentially dangerous or even fatal problem.
Any tool or appliance that has a three pin connector which includes a ground pin should only be used with a grounded-three conductor-extension cord and should only be plugged into a properly grounded outlet.
Never remove the ground pin so that it can be plugged into a two conductor (non-grounded) extension cord.