Everyone that has built a bigger site has had to deal with the mess CSS so easily become. There are ids and classes all over the place, and to find where a certain class is defined you usually need to use some search feature in your editor. Matching the other way, from the CSS to the HTML is even harder; you don’t even know what file a certain class is defined in. It’s a mess.
The Tree method tries to structure the CSS into logical blocks; blocks taken from the HTML.