I find the following quote from Chris Pratley (creator of OneNote) interesting "You know you have a good design when you show it to people and they say, “oh, yeah, of course,” like the solution was obvious.". No wonder OneNote is so heavily into literal metaphor of a notebook, which frankly does look and feel clumsy inspite they attempting to polish it up a bit off late (it used to look plain ugly till couple of years back).
From official Microsoft document: The user interface is specifically designed to look like a 3-ring binder, complete with tab dividers. Users can divide the notebook into multiple topics and subtopics for the various types of information they want to record and save. Workers have all the advantages of an old fashioned notebook binder with the additional advantages of being able to copy information from one section to another...
Document link: https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web...