When I got the Showcase Presents Superman Family volume 1 and noticed that Otto Binder did most (all?) of the Jimmy stories it dawned on me why Jimmy got his own book... it was so Binder could tell Billy Batson stories.
Really, Jimmy stands in for Billy, Supes stands in for the Big Red Cheese and the signal watch stands in for the magic word Shazam. The main difference between the stories is that Jimmy & Supes can talk to each other unlike Billy & Captain Marvel.