авторDefault methods have two important use cases:

Evolving existing interfaces. To implement the new Streams API, it was necessary to evolve the Collections Framework's java.util.Collection interface by adding new default Stream<E> stream() and default Stream<E> parallelStream() methods. Without default methods, Collection implementers such as the java.util.ArrayList class would have been forced to implement these new methods or break source/binary compatibility.

Increasing design flexibility. Abstract classes have traditionally been used to share functionality between various concrete subclasses. However, single-class extension has limited this design choice. Default methods offer greater flexibility because you can implement an interface at any point in the class hierarchy and access the interface's default methods from the implementing classes. Also, it's no longer necessary to create adapter classes for multi-method event listener interfaces. Instead, you can add a default method for each listener method to the event listener interface and override these methods as necessary.