First off, my iPad Pro is my only personally owned computer. I have a company issued MacBook Air that I have to use for work, but they also let us put all of our Office 365 software on our personal devices, so I can do some of that work from my iPad Pro if needed as well. (Honestly, I could do 99% of my day job from my iPad Pro if I really had to.) But as I've said in other threads, if I lost my job today and they took my Macbook away, I would not have any need to replace it with a personally owned Mac.
I have a huge mixture of both commonplace and niche uses for my iPad Pro:
Email: Stock mail app for personal, and Outlook for work
Productivity: iWork for personal docs (on the rare occasion I need to make one) and Office 365 for work.
Note taking: stock Notes or Notability for personal, OneNote for work. I LOVE OneNote and Apple Pencil together. Microsoft has done some great work here. Notability is fantastic too for more robust multimedia type note taking. I really wish I had an iPad with Notability in college--would have been a life saver.
Podcasting: I use RINGR for recording the conversation, and Ferrite for editing, mixing and publishing the final shows. This actually requires the use of an iPhone in tandem with the iPad, but I still think it's great that I can do this without a Mac and with better results than I ever got on a Mac.
Music composition: Notion--another app that is surprisingly well fitted for Apple Pencil and very robust.
Scanner Pro or stock Notes for scanning/signing/marking documents
Printer Pro to print to my non-AirPrint model printer
I'm sure there's other stuff I'm not thinking of for work.
For entertainment--
iBooks for reading books. I also subscribe to a magazine or two.
Playstation Vue--we use this as our cable TV and I have it tied in with the TV app and single sign on as well, so I can use it with all the channel apps and more easily look up On Demand content in the TV app. I also use the TV app for our movie library to watch things we've purchased in iTunes. We supplement with Netflix and Amazon Prime Video as well.
Apple Music for music. Pocket Cast for podcasts.
Home app, IFTTT, and Workflow for all my home automation stuff
odds and ends: Twitter, News, Civilization VI, TurboTax (I have done all of our tax returns on TurboTax for iPad for the last several years, and I love it.)
The iPad Pro is the only "computer" I really need anymore. I have never been particularly fond of laptops, and don't really use my work MacBook Air as a laptop--it's docked on my desk and hooked up to two monitors and a Magic Mouse and Magic Keyboard.
Because the iPad Pro is a fantastic tablet in its own right and can double as a laptop via the Apple Smart Keyboard when I need it for long form typing (though more often lately, I've been ditching the ASK and just typing on the screen), it's really more versatile, useful, and entertaining than any laptop or desktop computer I've ever owned. Add in the Apple Pencil, and it's an entirely new type of computer altogether.