I don't use OneNote much on my iOS devices, but I use it a lot on my surface Pro 3, a real computer . I use it on my iOS devices mainly as a viewer/ reader. They're synced via OneDrive and/or iCloud.

I use it in business as a note taker, and when recording is permitted it can also enable me to search notes and go directly to the point in time in the audio that the note was written. My handwritten notes are automatically converted to searchable text and indexed, while still being displayed in my native writing, and having full pen support enables hand drawn diagrams and shapes, erasing, etc. The paid version includes the ability to access the device's cam to embed photos and video in a page.

I tried Evernote also but it didn't support Windows at the time, and I wanted to search and display my notes on every type of device I use, including my office desktop computer. By using an app instead of paper it's more secure, searchable, and I don't worry about losing a paper notebook.