Well, partially I’d say it’s a marketing gag, or you could also say an opportunity for you to support Purisms work even more, because it might be cheaper to get the screen elsewhere.
As this was first an option during the campaign, I viewed it mostly as an option to show your support. Now, I’d see it more as advertising the possibilities, but yes, then it could be explained a bit .
@2disbetter might also have a point, that it’s still in development, so it might not yet be known what exactly works. On the other hand, all desktop apps (LibreOffice, Gimp) should just work out of the box, the hardest part is to make a smooth transition between phone and desktop mode.
I also think some information was just lost during the website transition. So, for your convenience, here’s a quote from the original campaign page:
Enabling the path for a true convergence device, capable to work as a phone, making video and audio calls, encrypted messaging, email, web browser, that can also become a full desktop computer with an option for a compatible keyboard, mouse, and monitor. It can be a desktop computer and phone all-in-one.