isolated grounds in branch circuits? How does this work with conduit?

Question

I recently was told that grounds from different branch circuits in a box need to be isolated.  I'd really like to understand the purpose of this. It doesn't really seem like it would contribute to either function or safety.
This also seems like it would imply that metallic conduit, which must be grounded can only have one branch circuit, regardless of size.
Now I know the latter isn't true, because NEC 250.122(C) gives an example of doing just that.  Is there any way of explaining this discrepancy, or is this just one of those things I just have to accept at face value regardless of whether it makes sense.
NEC 250.122(C):

(C) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.10(B)(1)(c).

manassehkatz-Moving 2 Codidact · Answer

I'll bet what you heard about isolating was about the grounded wire, which is what we normally refer to as the neutral wire. See:
Grounded vs. Grounding 
for a more complete explanation.
Neutral wires indeed need to be kept separate for different circuits in the same box. The exception, sort of, is an MWBC where one neutral actually serves two different hots on a pair of circuit breakers. Mixing neutrals can result in a situation where a neutral gets overloaded because another neutral connection broke, where if they were not mixed that circuit would instead go dead. Plus mixing neutrals will not work with GFCI breakers.
The grounding wires, aka "the ground", can be mixed without any problem.

isolated grounds in branch circuits? How does this work with conduit?

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