When you look at a beach ball, you notice its made up of triangular sections that meet at the points to form a ball when it's inflated.
The dart at the top of the puppet head pattern serves the same purpose, more or less.
Its the one fashion available to conveniently make a flat material conform to the shape of a ball (or as I like to call it, a complex curve).
If you examine the surface of a globe, you'll see many many darts in the paper surface. They do the same thing for the globe maker. They allow him to paste a relatively rigid material (paper) down to the surface of a ball without having super ugly wrinkles all over it.
The dart in the puppet pattern changes the shape and allows the foam skull to assume forms that it wouldn't normally.
-Gordon (who knows he's verbose... cut him some slack!)