neutral and ground in subpanel

[AKwiring]

Can someone clearly explain why the neutral and grounds need to be separated in the subpanel? How it could cause the breaker not to trip?
And how these same problems don't happen in the main panel where the grounds and neutral are together?
Thanks- I'm an apprentice and cannot seem to get a clear answer on the WHY

 

[charlie]

If the neutrals and grounds are tied at a downstream panelboard, the return current will split and some will return on the grounding system. Since the grounding system could be conduit, building steel, etc., the water pipes, gas pipes, and other things that people will come in contact with will be in the current return path. This return current will also mess up computer systems. Remember, the neutral is a current carrying conductor and the grounding system is only for fault current return.

 

[brian john]

And will affect the operation of the GFP, result in higher than normal EMF's, noise on sound systems, computers as mentioned. High enough ground current could result in different of potential in ground points, arcing in places where arcing should not occur when grounded surfaces are brought together.

Oh and it is an NEC violation.