The only use-case I can think for the Pinephone keyboard is if for some reason you want a physical keyboard to use while walking.
It's obvious: These keyboards are, or should be, primarily made for mobile people, so high quality and tray or coffee table compatibility are a must. The Psion 5-series keyboards are ideal for this as long as you don't fuck with the formula.
a benefit of the keyboard case is the battery it has that extends the phone's battery, which is nice, but it's an odd setup too. the charge controller on it was originally designed for a power bank and seems like a bit of a crude solution that's lead to multiple issues, like how you could damage the phone having it plugged into USB while the keyboard case is connected. it's slow to charge too, often it can't even keep up with the phone during use.