It saved your muscles from the very tiring work of agitating the contents of the washtub.
On the show Pioneer House, the drudgery and physical toll of hand-washing clothes was the #1 complaint.
as someone that has washed his clothes in his bathtub in college.....i agree
For which you might use a tool called a 'posser'
https://en.wikipedia.org/wiki/Posser
They are weird devices, if you didn't know what they are for you (someone) might never work it out.