Air driver used to be a stepping stone to becoming a full time driver.
You got the same support & training while you worked as an air driver, { actually you got no support , you were taught how to make stops on the blind } but plenty of time to learn all the alphabet soup that they now require and how to use a DIAD. You would do it until your time to go FT came up, the longer the better. Because you gained more area knowledge. Area Knowledge is VERY important. { Back when they started the classification in the late 80's , starting rate was $10/hr and 10 cents / mile if you used your own vehicle }