I suppose the term "fake" in this case might refer to the notion that you use the book because you don't know the tune by heart. I played with an old Virginia fiddler many years ago; Cy probably knew two or three hundred tunes, but didn't read music so the idea of having tunes written down was rather novel to him. And of course the book version was never exactly what he played, either. But for me, a "fake" book is a good place to figure out melodies and chords that take a lot of trial and error without some sort of reference.