The Temporary Bookmarks window is used for storing per-session bookmarks or bookmarks that you don't necessarily want stored in your main bookmarks file. It's especially useful when used as a To-Do list of URLs that you want to visit.
The Temporary Bookmarks window can also be used when recovering from a crash. By default, a session being restored after a crash will open links in new windows. If you prefer, you can choose to have the links added to your temporary bookmarks window, instead. Check the preferences section for more information.