Here is a taste of what RDF can be used for.
Classic Mozilla uses RDF extensively in the construction of the Classic application suite. Some of those uses involve RDF files stored in the file system, and some do not. Uses that do create RDF files are
User choices for window arrangement and position
Content of the Mozilla Sidebar
Manifest files for JAR archives, chrome packages, skins, and locales
Overlay database for application overlays
Search types for the Smart Browsing Navigator feature
Searching and Viewing states in the DOM Inspector
The Download Manager
In addition to these uses, Netscape 7 creates and uses many custom RDF files. Enhancements to the Classic Browser, such as those at www.mozdev.org, might also ...