Why Kamen Rider Ryuki has fillers?

Well, episodes 29 and 30 are fillers, but I don't see any reason to their existence, since fillers are made when the author needs time to develop the story so it can be adapted into animation (or any other form of adaptation). As far as I know, the Kamen Rider series are original stories, so they aren't adapting any current material (except for the Ichigo and Nigo).

If you don't have a factual answer, it can a theory.