Originally Posted by Harmon

If you'd ever seen an American chef prepare it, after the dough has been mixed, kneaded and has risen, the dough ball gets formed into a flat disc in the air by rapidly spinning it and flattening it with the chef's hands. The round spin imparts a circular shape on it.

In Italy they will toss them, spin them and then reshape them into a rectangle or square and cut them in square pieces to serve to guest.
Also when the pizza is round,the heat will be spreaded evenly...