For the life of me, I don't know why Planet is wasting money on developing these apps...
One reason is probably so they can offer better support for keyboard shortcuts. As the keyboard is pretty much their claim to fame, they probably want to make the most out of it. Also, as K-9 mail is a community project, Planet Computers may want to maintain a fork of their own, giving them more wiggle room, should some change in K-9 cause issues with Gemini and/or Cosmo devices. Perhaps, but that's just my guess, Planet Computers intends to contribute back to the official K-9 too, because it could be a reasonable thing to do, if you intend to recommend (a version of) it to your customers.
Another reason, maybe, could be that Planet Computers wants to have a set of productivity apps that are know landscape-friendly, visually reasonably consistent and uses the same keyboard shortcuts for the same purposes. Rather than coding it ALL from scratch, it could make sense to, where available, create tweaked forks of existing open-source programs. Now, again, I'm guessing here, just to show what the thinking COULD be like.
There's nothing wrong per se with that idea but it seems such projects should have lower priority than getting Linux fully operational, updating Android, and properly getting Lineage and Sailfish off the ground. LXQt is severely limited compared to GNOME and KDE. You still can't use the earpiece in Linux, etc.
K9 actually has all the same keystrokes, including Ctrl+Backspace to erase the previous word. In Airmail, this inexplicably changes the display font size.
I would much rather have fewer Planet software products if the ones that were released were of better quality.