Can I just add my experience here (iPad Pro, 10.5")
I too, debated whether the Pencil was a useful addition, or - frankly - an overpriced gimmick. But for me, I have found a use-case that has opened my eyes to what the combination can do, and I with an (almost) epiphany-like "where have you been all my life" and am loving the setup.
I work as a Security Consultant for a large corporate IT department. This often entails project-based workshops to flesh-out requirements on a whiteboard, where customer-departments sketch out their needs, and then I somehow have to make that work within the guidelines of IT security.
This - usually - used to involve collaborating over a whiteboard, drawing something up, then taking that drawing away and transcribing it by hand in to Visio, then going back to the customer with an "will this work for you?" answer (often: "no, because XXXX"), so, rinse and repeat.
My solution - absurdly simple. Draw up requirement on Whiteboard. Photograph Whiteboard with iPad Pro camera. Go away, review and come up with speculative design, having loaded photograph into "Paper" app (other apps may well be available) and annotated photograph with Apple Pencil, including any notes, questions, "blocking" issues. Send annotated picture via email to customer. Get feedback, review and repeat. Quick, easy, simple.
Once changes are agreed, I then draw up a "formal" Visio diagram.
Using the iPad is frankly saving me about two hours per project-request. And that's brilliant.
(just my use case, and I'm sure someone will pop up and say "ah, but, why didn't you do .. XXXX" but this workflow works for me, it is portable and most of all it is fast. And customers like fast).