It's pretty normal for original anime to get adaptation into other media like manga and novel, for example Madoka series and its 3rd movie Rebellion also got manga (straightforward adaptions with barely any changes for the plot). I think it's because tv series will be streamed on tv, and movie being on theater for a period of time before they stop and start selling Bluray Disk thereafter, so those adaptation might serve for PR. But well, CPK is not a theater release movie, it's a Netflix one, so yeah this feels rather redundant.I actually wonder why do manga adapt from that animated movie anyway. And I even want to know if the storyline and the vision for the movie and the manga (I hear there's a novel also???) is going to match up.
Is it because the movie is technically a "pilot episode" of sorts? And now they get to continue the story with another media or something?
And most importantly, will there be extra content?
It would be kind of a letdown if it's simply "adaptation" like it's from a manga to an anime. Because from an actual high-budget film to a manga, there will be many cuts on content and details. If that's the case, then one would be better off just watching the film from the start.
As for extra content, we will have to wait and see. Though for this way too fast pacing of the manga, I doubt we would get some. That said, I just peeked a little at chapter 3 raw, it does give a bit different touch like showing the brother irl learning about the newcomer group Kaguya/IroP, and that he quickly realized the one in the fox suit was his sister, but still not really "extra content".