This is pretty the same if you are familiar with expand/collapse on 
function/method in IDEs or editors for programmers (Kate, KDevelop).

When you don't edit method/function, you can "hide" the content leaving just 
name of the function visible.

The same goes for articles, books -- you don't edit too much at the same time, 
so it would be useful to collapse several sections of the book, and focus on 
2-3 (just an example). For collapsed section, you would only see the content 
of the heading, but the rest would be there but hidden. This way document 
would visually become shorter, important parts of text would be easier to 
grasp/edit.