I'm a digital artist/designer, and it's unimaginable that I do my job day to day on only my Mac Pro as well. I require other tools and media. Being a "Pro" device doesn't have to mean it's all encompassing. It means it's targeted toward professionals, particularly those who can take advantage of the Apple pencil to create notes, sketches and paintings. From engineers to salespeople to artist, and others. If you don't require such a device, you won't fully appreciate the value of this tool.
The iPP fits into my workflow nicely and expands my creative options. A pro tool doesn't need to do everything, but what it does do it needs to do well, at a professional level. For me, the iPP meets this criteria. It's become my primary device for expressing creative concepts, drawing storyboards, and digital painting. I still use my desktop computer for other tasks, but this doesn't diminish the capabilities of the iPP; they all complement my work style. Being a professional, you need a wide assortment of tools anyway.
I'll admit when the iPP was first announced, I was skeptical that a "Pro" device didn't feature a full OS. But then I realized I don't need the bulk of a full OS on my tablet; I already have that on my Mac Pro and MacBook Pro. I didn't want to buy another Mac computer but in a different form; I wanted a different experience, and my 12.9 iPP offers that.