There are many ways to link to non-XML content within XML, including binary content. Sometimes you need to roll all such external content directly into the XML. Data scheme URIs are one way to specify a full resource within a URI, which you can then use in XML constructs. In this tip, Uche Ogbuji shows how to use this to bundle related media into a single file.
I also touch a bit on unparsed entities and notations in this brief article.
Side note: Of course URLs are a subset of URIs, but I did want to mention that I prefer to use the term "URI" for the data scheme because it feels to me much more of an identifier-by-value than a locator. (I suppose it could be considered a trivial locator.)