Because it's required by the code. If you're within the same structure, all grounds need to be bonded together. If you're talking about a sub panel in a separate structure, the later codes for all systems (and the earlier ones when there is any other metallic connection between the building, e.g., phone, piping, cable) requires a ground as well as a grounded conductor.

The ground rod(s) and the equipment ground from the main panel serve different purposes:

1. The ground rod (earth ground) is capable of dispersing low-current/high-voltage surges -- typically lightening strikes in the case of a remote subpanel. Due to the high resistance of soil, however, it is almost useless when it comes to tripping a breaker in the event of an overcurrent situation (e.g., a ground fault in an appliance or other piece of equipment). A typical ground rod is capable of providing a return path to the center core of the power company's transformer for only about 5 amps or less.

2. OTOH, the equipment ground from the main panel is bonded to the neutral at the system bonding point and provides a good low-resistance path to the transformer. That's the path that will cause a breaker to trip in the even of an overcurrent situation.

Thank you terribletom for answering my question!

One more question... At what level does voltage drop become a concern? i.e. if I start out with 120 volts at the main, how much of a loss is acceptable before it becomes a concern (loss of power, motors running hot, etc.)

Thanks again!

3% is a typical max for conductor drop.

Since it is normally computed at full rated current it is very rare for it to actually happen (adding some additional margin).

Thanks brickeye... I'm considering running 6/3 copper to a barn 130 feet away (to the sub panel) which puts the voltage drop right around 3% at 60 amps. Sounds like I'm ok then!

Make sure you account for both directions (current out to the panel, and then the return current).

Some SW is 'smart' enough to double the distances using the same gauge, others need each side computed separately (especially if the wires are not the same size).

I used a calculator I found on the internet here

http://www.csgnetwork.com/voltagedropcalc.html

I will calculate manually to double check. Thanks again!