It's always puzzled me why a musician should bother to write down tunes that he/she was familiar with, like "speed the plough" or "soldiers joy". It would make sense to take dictation of an unknown tune from another musician, but not a familiar one. So the suggestion that less experienced players, especially those not blessed with a good memory, would be guided by the dots, would provide a good reason for writing them down; and the same pattern can be seen today. Of course this presupposes that musicians were more musically literate than they are currently given credit for.