This is one of several grounding methods that predate the widespread adoption of NEMA 5-15 devices. (Washers were among the first appliances to require equipment grounding.)
Another method is the cold water hose with steel wires braided into the rubber. It was specifically intended for the cold side. (This method wouldn't work for modern machines, which have a plastic solenoid valve body. It wouldn't work with plastic pipe, either.)
My dad told me he remembers when some towns expected washers to be grounded with a jumper as shown here. Likely as not, few users understood this, and when machines were moved, the ground was not reconnected.
I've heard that some jurisdictions, before the '96 NEC, required this type of jumper for dryers. The internal bonding strap had to be removed, and the NEMA 10-30 cord freed of a grounding connection. Imagine what happened when such a dryer was moved to another house... No more ground! I suspect the same happens when today's washers are moved from a NEMA 14 to a NEMA 10 location.