As a writer, I often need to design new monsters, spells, or magic items for certain encounters.
For example, for "The Village of Oester", I wanted to create a demon-seductress with the upper body of a beautiful woman, but the hideous lower body of a serpentine, octopus-like creature (hiding this portion behind brush). There is no creature like this that I know of, especially anything OGC. So I created it and of course, when you do that you need to describe it in the back of the product.
The same goes for a new spell called perception which was in a potion used by a baroness who found out her family's fate was doomed and then promptly committed suicide. I also wanted the antagonist of the story to wield a special magical sword and that needs a description in the back, too.
I'm sure some new rules are included as just a "bonus", but a lot are required for explanatory purposes.