Making Dough: Preferment + Cold Rise - Necessary?
I have not tried it but I suspect it is just as you say, the sponge phase allows gluten to develop and improves the texture of the resulting bread.  Brioche should not be fluffy like a cake but should have a little bit of stretch and chew to it, but should paradoxically not be tough either.  It also needs a sour element in the flavor to offset all of the fat and sweetness.  The best way to achieve this is with the long slow process, which internally creates long, supple strands of gluten rather than smaller, firmer ones.

Another trick with brioche is keeping all of the fat (usually butter) in suspension in the dough throughout the process so that it does not leak or sweat out during baking.  The sponge, fermentation and long overnight rest develops long strands of gluten which form an internal mesh that keeps the relatively heavy dough in suspension.  If the dough is prepared too quickly it will "sweat" or the fat will liquify and leak out and you'll lose all of that lovely butter.