May I refer the Neophyte to folkiedave's anecdote.
Classical playing and folk playing differ. Some instrumentalists can do both. Classical playing tends to be very score-dependent. Folk playing may do without dots altogether or use them merely as a rough guide.
At extremes, a classically-trained player may be unable to play be ear, at least on first immersion in a session, while a folk player may be unable to read a score.
It may also help to consider the instrumentation and personnel required for what may be an orchestral setting.
I'm fairly sure that the orchestral pieces are still done once in a while by classical ensembles. I'm equally sure that if a fiddler [perhaps with others] should do Greensleeves followed by Lovely Joan, or whatever the sequence is, it may coincide with the tunes in a particular classical setting, but won't be that setting.