You need them to take the next step, it's like a butterfly effect, in a few years time, you may not need them, but they are the choices you make that decide whether your future is utter crap or not, each choice matters, so for the love of god make multiple plans for the coruses you take and do very good research on the job you want to take.
Good GCSEs lead to wider choice and variety for A-Levels. For specific Uni courses you do need specific A-levels which you can not acsess if you dont have enough high GCSE grades. For specific Uni courses you do need GCSEs in the 6's and above to be considered. Everything you do plays a part in your future.
All of this isn't considered once you're in your 3rd or 4th job because they are stepping stones to getting a job in the first place. After your 3rd or 4th job maybe, experience matters more. If you're considering doing A-Levels use the summer to really push yourself because to redo them is incredibly expensive and generally not worth it, neither is the stress of clearing.
Russel universities serve as a direct pipeline to higher ranking establishments because that's where they like to hire from first before considering other Fresh graduate applicants, especially finance.
Essentially good grades -> good uni course/uni-> higher chance at a better job if you're in a place where you can secure high quality connections that you probably won't find at lower ranked unis, better starting point for your route into the future.
Bad grades -> low ranked uni courses only available -> end up in a bad position, 27k wasted which you're forced to pay, and take a different route.
There are other routes, like starting your own business young or just straight up working, but if you're going to uni you'd better have a very good plan, or it will not work out...