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.
That was my suspicion initially. In fact, when I read about mass DNG adoption, my first thought was "but how would it work for this company?" (admittedly I don't know much about DNG, but intuitively I had my doubts).
And then I saw your comment.