It was also a great way of having an apprenticeship program by which everyone had the opportunity of being trained to go up to the next level; or it became apparent that a person could not handle the next stage and he/she'd gone as far as ability would let them. (The fact there was studio politics that stood in the way of deserving talent does not diminish the validity of this team concept.)
Before there were animation schools, on-the-job training was the only way to learn more than just the basics (which you can teach yourself). Even today, an apprenticeship program would be the best way of developing animation talent. But that would mean a commitment by the studio to keeping its artists employed from production to production.
The following is a basic Old School Unit the way I remember it on the feature Raggedy Ann & Andy:
2) Rough Inbetweener
3) Assistant Animator
4) Breakdown Assistant
5) Clean Inbetweener
The Animator draws the rough keys and eccentric partials. These are shot for a pose test. When the pose test is okayed by the director, the scene goes to the Rough Inbetweener who rough inbetweens the entire scene for the first action pencil test.
Once the director okays the rough pencil test, the Animator may go back in and add the important lip sync (if he hasn't already included it in his rough pass). The Assistant Animator then cleans up the keys, making adjustments so that the character is on model, and checks the exposure sheets to make sure that any leveling of the figures, overlays, etc. are correct.
The Breakdown Assistant then cleans up and completes any eccentric partials and crucial inbetweens, leaving the Clean Inbetweener to finish off the rest of the drawings.
But let's not come away with the mistaken idea that working this way was a bed of roses. The pressure on the artist to produce has always been enormous. The following is a 1976 cartoon by Eric Goldberg showing a bit of foxhole humor during my last few days on Raggedy Ann and Andy. I was leaving to start my first gig as an animator at Nelvana...and there was the assumption that I would be introducing Hollywood animation production methods to that studio in the Great White North.