It was also a big deal to make a film like this. Today you can just prop up your phone on something and go to town, but these videos would have been a huge amount of work. Once you're putting in that much work, you had better make sure the result is good.
This 1937 video about differentials is also enlightening:
http://www.youtube.com/watch?v=yYAw79386WI
Why is it that old tutorial videos are so thorough and informative?