I use my iPads for most of what I do ... it is more the range of what I can do in various locations that is impressive to me rather than the actual tasks.
I have done some music on iPads with various apps when sketching ideas away from home, though I do prefer the MBPr with 24" screen when finishing these things at home. I also use my laptop for backing up phone and iPads, putting music and video onto them, and printing. Everything else on iPads:
Communications to various countries ... messages, email, Skype, FaceTime.
All office things.
Writing (which is most of my work, journalism and fiction) .... IA Writer, Pages
Research.
Studying and learning various things ... literature, cosmology, history, futurism, technology via books, courses, lectures, and learning art, with Apple Pencil, and better photo editing (Affinity), a broad range of health stuff.
Apps for... cooking, workouts, yoga, gardening, etc, etc.
In short, I almost always have the Air2 cellular with me in a small bag with KeysToGo and a light stand, I can work (or play) anywhere, with everything important on it or with access via 3G to cloud. One of the big benefits, even though my MBPr is portable enough it doesn't have 3G and I don't like using public wifi.
Another big benefit, comfort, less sitting at a desk, can lay on sofa for reading, research, study and also to write with Apple Pencil in Notability or Nebo (coverts to typed notes really well), or annotate PDFs,
Or to type this with one hand with onscreen keyboard.