This hidden page is used for uploading static assets (files/images/etc) that you can link to elsewhere within the site. Simply type something, highlight it, and click the "chain" icon above (next to Bold/Italics) to create a hyperlink. Once the dialog appears, select the "File" tab, then click the up arrow to upload your file. Once uploaded, you will see your highlighted text has become a hyperlink. Click it to reveal the pathname to the asset (usually "/s/<file-name>"). This is the relative URL that you can use in any other hyperlink on the site to reference that file.
Site Code Assets
Externally Linked Content