Large sites can have many pages, images, script files etc, therefore it can be useful to see how everything links together and see an overview of your site. The Site Navigation View is centred around the current file being edited. It allows you to control the depth of links that will be displayed, scroll around the view, and even zoom in / out so that more / less of your site is displayed.
Just viewing the linkage is useful, but you may also decide you want to edit one of the pages in the view, this can be done by simply double clicking on the file in the view. A popup menu, identical to that available in the Site file list is also available by right clicking on the file in the view.
Processor usage will go to 100% sometimes, just quit Screem and restart it.
Sometimes links will stay in the Checking state, this is a problem with gnome-vfs