Use Cases and Deployment Scope
OneNote serves as a personal notebook to make sure that you are able to make notes.
It can come handy in many ways.
While mostly it is for notes relating to work, for example meeting notes, plans, etc, but more importantly, it also serves as a safe and secure place to allow you to add more personal notes, whether they are notes from one-on-ones with colleagues, or general personal planning.
It also helps that there are apps for nearly every device, be it a Windows machine, a Mac, an iPad or your phone.