I am struggling at how to convey this quite properly but: It's visually hard for me to tell which notebook i am in when I'm in a subnotebook or further down without it. So being able to navigate up without having to search my screen/overall notebook list would be helpful for quickly going up/knowing where i am.
For reference:
And as a really quick example:
Where '...' is a link to the main/parent notebook and 'sub notebook ^' would be one up from this current notebook/file.