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.