Chris Aldrich wrote some nice articles what kind of applications you can achieve with the help of webmentions:
http://boffosocko.com/about/website-philosophy-structure/
http://boffosocko.com/2017/12/15/threaded-replies-with-webmentions-in-wordpress/
http://boffosocko.com/2017/12/24/adding-simple-twitter-response-buttons-to-wordpress-posts/