All household grounds must meet only at a common point for reasons
similar to why ground loops create hum in a stereo system. Neutral,
equipment ground, and earth ground that connect at a common point (main
breaker box safety ground) avoids 'ground loops' and other adverse or
surprise currents.

To explain same using a different perspective, first, all wires are
electrically different at both ends. That difference increases as more
current flows. To better explain this, we express that difference as a
separation or electrical distance.

A three prong appliance is powered from wires that are distant from
the breaker box (because they carry current). A separate safety
(equipment) ground wire connects directly (shorter) to breaker box
safety ground because it carries no current. Appliance connected
electrically shorter to breaker box means greater human safety.

Again, if safety ground and neutral wire were connected anywhere
(other than in breaker box), then that safety ground wire would be
electrically farther from breaker box (because it carries current).
Another perspective that explains why NEC demands separate neutral and
ground wires.

Another situation: suppose neutral and safety ground wire were both
carrying current. Suddenly that common wire breaks. What happens to
appliance connected to third prong safety ground? It suddenly becomes
electrically hot - directly connected to black hot wire. AND no safety
ground exists to protect human and trip circuit breaker. We want
neutral wire separated from safety ground so that any neutral wire
break always leaves appliance still connected directly to breaker box
safety ground and not connected to a neutral wire that is no longer
connected to breaker box. Just another reason why those two wires
always remain separate.

Home has its own single point safety ground inside breaker box.
Power wires connect that system to another system that has its own
single point ground - pole transformer. Pole transformer connects
primary (high voltage) ground, secondary neutral, and earth ground to a
common point. Lightning strike to primary (high voltage) wire simply
gets conducted safely to earth at transformer which is but one reason
why that primary wire can be highest on pole.

Meanwhile, household single point ground in breaker box is one ground
system centered at a single point. Transformer has its own single
point ground system. How far apart are those two grounds? As current
increases on neutral wire (transformer to house), then both grounds
become electrically more separated. Again, using a perspective of
electrical distance to explain a concept.

wrote:
I installed a subpanel when I switched from an electric stove to gas. I
used the 40A 220V breaker that formerly served the stove to power the
sub panel. .... The grounds and neutrals all share the common bus bar
in the sub panel. Everything has worked fine for years now. Can
someone explain why I read that ground and neutral are to be isolated
in the sub panel? Please don't answer because of the NEC since
that does not explain why. What is the risk of my current situation?