The GNU toolchain very much assumes the use of character input (e.g. a keyboard) and output (e.g. a screen). That's an assumption all digital computers preceding it made too (as far as I know), but I don't know a way to drive groff using a touchscreen, short of it emulating a keyboard, badly.
I've always liked keyboards on my phones, for data entry and messaging needs. It seems to me that if I can use GNU/Linux on that hardware and it's properly supported, that opens up so many more uses for the device than any other mobile OS gives me, but it does need a certain amount of learning to get the best out of it, so it's clearly not for everyone.
The thing I'm looking forward to most in the Pyra is its keyboard actually. The other day I was doing a little HTML coding, and closing an anchor consisted of left trigger-comma, fn+c, a, left trigger-dot. Forgetting that a symbol was shifted rather than accessible via Fn and hitting Fn meant either hitting it twice more or pressing they key then deleting the wrong symbol. Even in the best case it's quite the handheld gymnastics entering all that.