It also shows that you had the persistence and self-motivation to work hard on a big project for multiple years and are able to learn a new topic to mastery and

It also shows that you had the persistence and self-motivation to work hard on a big project for multiple years and are able to learn a new topic to mastery and further. Sure, you are then also the world's leading expert on <extremely narrow topic of your thesis>, but most of all a PhD shows that you are able to do research.