Yeah, Duet is nice. If you run Alfred, it can be similar to having the TouchBar because you can customize tasks and just hit a button on the iPad.
I like to use mine with SimpleControl Remote to control music, what is playing on my mounted TV, etc. If you have the iPad Pro, Astropad lets you edit Photoshop photos with the Pencil. I also like to have open my security camera app on the iPad while I am at the computer. Anyway, those are the first to pop into my head.
The main reason I use my iPad more than my MBP or iMac at home is simply the form factor.