How Dave Winer Used OPML for Lists of Feeds

Reading Lists
  Instead of manually subscribing to each RSS feed individually, users could download an OPML file containing a collection of feeds. This OPML file could be imported into an RSS reader, which would then automatically subscribe the user to all the feeds listed in the file. This was a convenient way for people to share and disseminate collections of their favorite RSS feeds.
Directory Structure
  Given the hierarchical nature of OPML, it could represent not just flat lists of feeds but also folders or categories of feeds. This was particularly useful for organizing large numbers of feeds or for curating lists around specific themes or topics.
Interoperability
  The use of a standard format like OPML allowed for interoperability between different RSS readers and software. If you decided to switch your RSS reader, for instance, you could easily export your list of feeds from one application and import it into another.
Sharing and Collaboration
  Because of the simplicity of the OPML format, it became easy for individuals and organizations to publish their own reading lists and for others to collaborate on curated lists of feeds.
Extensibility
  OPML's structure allowed for the addition of extra metadata, if necessary. While the core purpose was for outlining, its flexible nature meant it could be adapted and extended for various other purposes beyond just RSS feed lists.