Making Dough: Preferment + Cold Rise - Necessary? I'm making a brioche. Currently, I do a sponge &amp; dough process + an overnight cold rise. I'm wondering if the sponge and dough is necessary if already doing a cold rise?

From my understanding, the sponge &amp; dough gives the gluten a chance to develop without having to knead the dough as much. Wouldn't a cold rise with occasional folding do the same? If so, is it overkill to do both steps?

My current process for sponge is: I take part of my flour (33%), liquid (33% milk), 1% sugar, and 1% of 33% yeast - and give it 2 hours.
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.