When I want to focus on a very small part of my work, I find very useful to be able to open a window that I can resize as I want. And when I close that window, I find again my text in the place I wrote it. A window is for me such as an opening on your work, as if you looked at it with a magnifying glass. But I think that the metaphors we use to describe our usage of text windows are as many as we are.
The Text Window (opened from the contextual menu or ⌥⌘X) is useful to have when you are browsing a map or timeline and do not want to have the normal text pane open.