I remember a washboard attached to the twin-tub and my mother remarking that she didn't really need it.
I recall laundry being done by hand before the twin-tub and, once we had an automated washing machine, my mother stating that she could discard the mangle.
Necessity (convenience) is the mother of invention.
These days, the galvanized baths are used for planting or contain ice and drinks at BBQs 😜