There are many reasons, of course, why someone might snap...

There are many reasons, of course, why someone might snap their fingers and grin. If you heard some pleasing music, for instance, you might snap your fingers and grin to demonstrate that the music had charms that could soothe your savage breast. If you were employed as a spy, you might snap your fingers and grin in order to deliver a message in secret snapping-and-grinning code.