I find plenty of uses for it outside of business or learning. I use it to just jot down all the minutiae of daily life that I want to remember. I have a Recipes notebook. I have sections for my cars - especially my older ones where there is a lot of bits and pieces of information that I find about them over the years. That section will have manuals, lists of parts sources, my to-do lists for repair and restoration, snips of forum posts I've found useful, etc.
I also use it as the digital repository for all the electronic manuals of various things I own - rather than have to manage individual files, everything goes into OneNote- manuals for my TV's, appliances, etc. When I'm feeling especially organized (or feel that something is of vital importance) I scan the receipt with my phone's camera and it goes right into my notebooks.
Whenever I am learning about a new subject, I aggregate all my knowledge there. Whether it's shopping for a car - where I can do my calculations, keep track of contacts, quotes, etc. Same for house projects.
I hate keeping track of paper, so literally everything goes into OneNote.
I think most people don't think they have a use for it because they don't have a clear idea of all the various ways they already track information in their daily lives. I've been selling OneNote at my workplace for close to a decade now, and while it's been successful, the initial hump is always convincing them of why they would want to use it. I usually let them sit on it a bit, and then as I watch them struggle to find things over time slowly their awareness grows and I use that as an opportunity to give them some real-world examples of its use.