The shade:shade Mojo is quite well documented, here especially about the createDependencyReducedPom parameter, which will create that dependency-reduced-pom.xml file: maven-shade-plugin/shade-mojo.html#createDependencyReducedPom
In short, this is quite useful if you intend to use that shaded JAR (instead of the normal JAR) as a dependency for another module. That dependency-reduced-pom.xml will not contain the JARs already present in the shaded one, avoiding useless duplication.