I actually had a very interesting talk with a student in sports science about exactly those machines some time ago. Don't know if it is true but according to him, science in the early 20th century did not really know how training muscles worked on a low level. Of course they knew that exercising creates a fitter and more defined body but the exact trigger and what happens in the body was unknown. Apparently a theory / widespread belief was that it is the movement of muscles itself that stimulates growth even if created by external forces. And that misbelief let to the creation of those (from todays view) odd training devices. They were used mainly by women since actually demanding physical exercises, not to mention classical sports, were still seen as inappropriate for females.