That, and for the sake of practice. To learn how to manipulate the weight of the quarter as it rolls across your fingers.

[He takes her hand with his own, turning it palm down.] Palm down. Relax your fingers; they should have a slight curve to them. Yours was too stiff. The quarter will only ever bounce off that way.

[Not that he expects this to make it magically work for her. But it'll make it easier, if she ever does get the hang of it.]