Most outlining programs afford the ability to write very intricately ordered deep structures. The temptation is to develop such a hierarchy at the very beginning of one's writing project, but this can be counter-productive. A flat outline, that is, an outline that goes no deeper than one or two levels is more than sufficient at the beginning of most projects.
The first outline should be a rough indication of all the things you need to cover and give you a route that you can follow. The details will follow later — as you are writing. A more hierarchical structure will thus emerge as you are working out the details. For that reason, a hierarchical outline makes sens only later, when you are completing the project, that is, the essay or the book. It is a means of tweaking what you want to say, not a means of planning. Both kinds of outlines have their place. Most people (in my experience) mistake the order, however.