Note: this is rather a tip for beginners, so people who know Favorites view can skip it.
From time to time it is useful to have access to files which are not a part of a project. I for instance use NetBeans to update the HTML pages on the netbeans.org website. Obviously, these files are not a part of a project, so how to open them? Simply use the Favorites view.
Favorites view is useful when you want to quickly open a file and take a look at it contents or for editing of files which do not need building, such as HTML. It can give you access to files which are not a part of your project. Although you can use multiple source roots in the projects view, it may not make sense for all types of files so favorites view can be used to add, view, modify or delete any kind of file anywhere on the disk.
Favorites view is not visible by default in the explorer, you can open it from the Window menu or by pressing Ctrl-3. Root of a new favorite directory is added by right-clicking in the favorites view. These roots are remembered accross sessions so you need to add them only once (unless you change the userdir).