Everything. I run a one-man engineering consulting firm, so it's used for engineering analysis and design (AutoCAD, RAM/Elements, numerous small utilities) and mudane reports, image id and markup, PDF reviews during the day. Mine is connected to two 42", 4k monitors so I can work in two D size (22"x34") prints at near full-size simultaneously. I've taken it out in the field and used the camera to take a picture, SketchBook to mark it up, and then emailed to a drafter to get drawn up in CAD. I used it a couple weeks ago for a telecon. I "dialed in" in a chrome window for audio with earbuds, a couple of users were participating via chat in a box, I had two working documents up - one in PDF, one in Word - plus a second chrome window with the live meeting minutes (that someone else was taking), and a OneNote window I could jot things down on...all while walking around my living room.
I do some hobby photography stuff on it, along with the occasional video (Adobe CC suite). I also compose/arrange for two vocal ensembles using MuseScore (still trying to get the hang of StaffPad) and use Reaper and Melodyne Studio to record and mix stuff for the two groups. I'm currently working on a mix for the younger group (mixed, 4 part contemp. a cappella), and it's got probably 100 takes, 20 tracks (including scratch, guide and click, plus downmix) with effects on several.
I've just been to a weekend conference where I used it to pull up sheet music from the past 10 years of events to help out where my memory was not 100%. I watched movies on the way home on the plane.
Like I said, I use it for just about everything.