I'm a bit confused. Illustrations in magic books are illustrated using a photograph as reference. If a photograph has already been taken, then why bother drawing it? Why not just use the photograph in the book? It's really been buggin' me.
Generally I believe line drawings are easier to follow than photographs. With photographs comes distractions that with line drawings you can't see. Simplicity is what it really comes down to.
Actually, Line drawings are not necessarily done from a photo.
Also, it is a stylistic choice. Some just want the look of illustrations, and they also allow for certain things to be shown that can't be seen in a photo, like making a cover card transparent so as to see what is going on beneath, yet still show the card.