Atom.io is a great tool for outlining when you utilize markdown and the ctrl+cmd+↑↓ feature
There is also an outline package available that puts a clickable outline on your sidebar.
I also recommend using the Markdown-Folding page so you can quickly toggle text under headers.
Outlining is an exercise to help us see the hierarchy of ideas and identify dependencies.
The outline becomes your table of contents if you want to publish your work. It provides the reader with a helpful navigation.