Just curious really.... if using the metal junction box as a place where wires are spliced....and technically the wires are connected and tied and all...what is the reason to ground that box ??

I mean ....there is no reason that the wires inside the box would become separated / disconnected so just wondering why ground the box ?

 

I don't know the answer, but I can absolutely see a few scenarios where the box could become energized but power isn't interrupted. Then later, someone goes back to do some work in the box and ZAP!

 

True, there is no reason that the wires would become separated. Reality is that it happens. Sometimes when installing the wires are not wire-nutted properly and exposed conductor touches metal.
Also they used to use tape instead of wire-nuts. The tape could dry out and if the splice was jammed against the box it could wear through with a bit of vibration.

 

so this is purely for safety in the unlikely scenario that hot wires...which are obviously connected via wire nuts and up to code (i.e. using correct size for the number of specific wires...)....would somehow got exposed .....

OK...not a lawyer but I can accept that...better safe than sorry...even if it's one in a million.

BTW...the real reason for this question started with me thinking why do we still use metal boxes and not plastic everywhere....which obviously don't need to be grounded

 

There are many homes with metal clad wiring and ALL commercial uses metal boxes.

When I do new house roughing I use predominantly plastic but I use metal for just about all my old work.

 

It happens more then you would think. I get service calls all the time to troubleshoot lights out calls, calls of a circuit breaker that keeps tripping, or equipment that is not working, and find that someplace in the circuit the wire insulation has worn through and is shorting to a metal box or conduit. I have even found a section of MCAP cable that had a internal short on a set of can lights on a building. If any of these situations were to happen and the boxes, cables, or conduits were not grounded, they would become energized and could injure/kill somebody or start a fire.

As PJ posted, commercial is almost always metallic wiring methods for reasons of smoke and fire. Much of our work is in plenum ceilings.