The shape of a brioche à tête, it may surprise you to learn, is primarily functional. When made properly (that is, with lots and lots of butter) brioche dough needs a large amount of yeast. Why? To lift all that fat off the ground of course! Such a high proportion of yeast causes an extremely aggressive rise, both before and after it’s placed in the oven.
And that presents a problem, for as the brioche bakes the exterior crust will harden well before the interior is done rising. If there’s no mechanism in place to accommodate that outward pressure, the inside of the brioche will push out through the crust causing ugly random cracks. This is of course the reason bakers score the tops of bread loaves. Though with a dough as dynamic as brioche, scoring merely presents little doorways through which these little breads try to turn themselves inside-out.
Topping the brioche with a tête, on the other hand, gives the bread some expansion space (namely, the little seam around the bottom of the tête). Rising dough from inside the tête pushes down, dough from the main part of the brioche pushes up. The result is still a region of cracking, but cracking of a comparatively controlled and uniform kind — and it’s all topped off with a big shiny ball! The impression you’re left with, in the words of the immortal Peewee Herman, is: “I meant to do that”.