I can't say for sure, but I need the sidebar tree view it helps one keep stuff organized, maybe it's only me and my "old brain"
Example I'd like to search for "put" in this case Bash related tip
Search result returned a few notes:
One for my wife (love-notes)
Another for my shopping list notebook
And only then that old bash note i was looking for Bash notebook.
Yes sure I could be more specific on search and probably narrow it down till i get to that Bash put command.
But hey I know this was saved under my Bash notebook, using sidebar I'd first move over to that notebook and search for notes only under that specific notebook.
Think of it this way notes and post-its have always had color or were organized per specific categories, this same order is kept with notebooks and the preferred method of accessing them as such is via tree view, rather then one search location that checks all of them together.