Annotating PDFs. Annotating and more annotating. And occasionally drawing.
Also, with the iPad 12.9" resting on a stand on the desk, it's slightly more convenient using the Pencil to navigate/interact with the screen, as more can be reached across the span of the screen, without having to move your arm constantly. Small mercies/1st world problems...
Like @c14nhl, I never use mine to draw. But all my notes are in my iPad. I have notebooks for every topic, and always have my information with me. I can also quickly add photos or .pdfs and make notes on them.
The thing that really made the pencil useful for me was the MyScript Stylus keyboard app. It's a custom keyboard that lets you use handwriting anywhere in any app where keyboard input is accepted. They recognition is fantastic, even for my terrible handwriting. If you prefer they also have Myscript Stack which lets you write character by character. Both are free and make the pencil a joy to use where I either can't or don't want to use they IPP hardware keyboard.
Another non-artistic/drawing user here. I use it a lot for annotating PDFs and making quick notes.
Actually I have one use case at work that I find interesting. I sit in front of an iMac during the day, and a few times each week I receive PDF documents that I used to print out, mark notes by hand, and then scan and put back onto my Mac for future reference/work. With the iPad and Pencil, I use iCloud Drive and open them quickly on my iPad to markup with the Pencil, and then within seconds those updates are reflected on the Mac. Saves time and paper and really makes it an interesting work tool.
Note taking. I make forms and save them as PDFs (previously I would have the forms printed off on card stock at Office Depot), and use the iPad Pro to 'annotate' (fill out) the forms at work. Using Microsoft OneNote, I can even search through the handwritten portions later if need be, and everything is accessible in one place, as opposed to hanging on to stacks of the cards each year.
I stopped using a notepad for meetings and now use Notability 100% of the time. I also download all my PowerPoint training presentations and workbooks into Notability and underline and highlight as I present. Also add in blank pages to use instead of a flip chart. I take ideas from the group and write on the iPad and project to the screen. So convenient to keep everything on the iPad. So many uses.