It’s the best place to add “signature steps.” Things like noise reduction, chromatic aberration correction, and one-step HDR processing.
I used to work for a camera manufacturer, and our Raw decoder was an extremely intense pipeline step. It was treated as one of the biggest secrets in the company.
Third-party deinterlacers could not exactly match ours, although they could get very good results.
But it was a pretty major one, and I ran their host image pipeline software team.
[Edited to Add] It was one of the “no comment” companies. They won’t discuss their Raw format in detail, and neither will I, even though it has been many years, since I left that company, and it’s likely that my knowledge is dated.
Can you share the reason for that?
It seems to me that long ago, camera companies thought they would charge money for their proprietary conversion software. It has been obvious for nearly as long that nobody is going to pay for it, and delayed compatibility with the software people actually want to use will only slow down sales of new models.
With that reasoning long-dead, is there some other competitive advantage they perceive to keeping details of the raw format secret?
They feel that their images have a "corporate fingerprint," and are always concerned that images not get out, that don't demonstrate that.
This often resulted in difficulty, getting sample images.
Also, for things like chromatic aberration correction, you could add metadata that describes the lens that took the picture, and use that to inform the correction algorithm.
In many cases, a lens that displays chromatic aberration is an embarrassment. It's one of those "dirty little secrets," that camera manufacturers don't want to admit exists.
As they started producing cheaper lenses, with less glass, they would get more ChrAb, and they didn't want people to see that.
Raw files are where you can compensate for that, with the least impact on image quality. You can have ChrAb correction, applied after the demosaic, but it will be "lossy." If you can apply it before, you can minimize data loss. Same with noise reduction.
Many folks here, would absolutely freak, if they saw the complexity of our deBayer filter. It was a pretty massive bit of code.
What you can store, is metadata that informs these "first step" filters, like lens data, and maybe other sensor readings.
One of the advantages to proprietary data storage, is that you can have company-proprietary filters, that produce a "signature" effect. Third-party filters may get close to it (and may actually get "better" results), but it won't be the same, and won't look like what you see in the viewfinder.