If you search on the Mac App Store, you will find dozens of Markdown note taking apps with a markdown pane on the left and a preview / reading pane on the right.
Many of them also feature a sidebar on the left where you can "conveniently" browse your files. Yay!
Yet, none, and I really mean none of them, offers an option to see an outline of your document in the sidebar!!
I have notes that are 30 pages long. They are living documents, with a structure. They make full use of 4 or 5 levels of headings. And scrolling up and down to find the relevant part just isn't practical. Having an outline would allow to click and quickly scroll to the relevant part of the document.