Making Dough: Preferment + Cold Rise - Necessary?

Making a sponge gives the flour, water and yeast a chance to react in the absence of salt (which slows down the yeast action). If you omit this 'autolyse' phase then I would not expect the outcome to be the same.

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.

Sponges and preferments can be used for different purposes, but the process you describe is not good at achieving any of them. So I would say it serves no discernible goal and you can abandon it. I would likely stop using the whole recipe - if it was designed with so little understanding of breadmaking in that important point, it is probably suboptimal in other points too.

You can find many good recipes which provide different end results and require differing amounts of effort. In that sense, if you pick a recipe that has a sponge, the sponge is only "necessary" in the sense that it creates the specific end product that the recipe intends - but if you omit that part, you will still end up with a brioche, just with a brioche with a different taste and texture. So, it is never necessary - but it might be preferable, if your personal taste includes a prefernce for the end product that comes out of a given recipe that involves a (well-designed) sponge process.

So, the way ahead here is to search for recipes, try them out and when you find one you like, stick with it. It is up to you to decide whether to consider recipes with a sponge or without a sponge when choosing the ones to try - if you are unsure, maybe a variety is best, so you can make an informed choice.