In article , pilgrim says...

If I fail to connect my electrical system to a water-pipe-ground, what
would be the source of the electrical shock that the plumber might
receive?


Any electrical appliance can have a short from a "hot" wire to its metal
case, and in-turn to ground. Like a furnace, washing machine, dryer,
refrigerator, garbage disposal, dish washer, electric water heater, etc.

Shorts of this nature I have personally seen...

-An electrical wire is connected to an appliance through a metal hole
with sharp sides and no "wire clamp" is used. The sharp edges of the
metal cut into the wire.

-An electric range had defective insulation on its internal wiring. High
heat caused the insulation to shrink back several inches and thus expose
bare electrical wire which then touched the metal frame of the range.

-High heat in a bathroom light fixture - just from the light bulbs -
caused the old wiring insulation to become brittle, crumble and fall
off, then the bare electrical wire touched the metal light fixture frame
and its attached metal medicine cabinet.

-An electric water heater heating element had an internal short to
ground.

-A garage door opener had a short to ground which energized the opener
and via that the metal garage door.

Just one strand of a stranded electrical wire can stick out and touch a
metal case. And electric motors are notorious for leaking electricity to
ground - they are many times open at the ends for ventilation and crud
can get in there which will cause a short to ground.

A modern "to code" grounding system will protect you and your family
from being electrocuted should any of the above happen. Thus do it right
and have the work inspected to be sure it is done right.

That is how these electrical codes come to be in the first place...
Someone gets electrocuted, then they come up with methods of wiring
which will keep that from happening again (if you follow the codes).