Neutral and ground seperation at a sub-panel
We don't build the system for normal circumstances.  If we did, we wouldn't need circuit breakers, would we? :)
Ever hear of a &quot;Lost Neutral&quot; situation?  This is where neutral becomes loose or broken, and is no longer pegged in the middle at 0V between the two opposite 120V phases (or at 0V in between the three 230V phases in Europe).   Neutral is subject to a &quot;tug-of-war&quot; amongst the various phases - the more load on a phase, the harder it pulls.  This causes the weaker phase's voltage to go higher than spec, which often burns out equipment.
Suffice it to say, neutral is no longer at 0V, and could be as high as 120V or 230V if only a single appliance is turned on (or still functioning).
Now, what would happen if the ground was attached to that lost neutral, i.e. the chassis of all equipment?  Well that would be at a high voltage along with the neutral, and people would be getting nailed off switch plate cover screws.
That's... why you maintain ground separately at all points beyond the main disconnect.