I get that you can generate a sample privacy page with the new GDPR update. I don't understand what it actually does? Is there an API endpoint or something? When you "select an existing page" what happens?
It will add a link to the selected page to wp-login.php and a link to it is included in the Erase Personal Data confirmation email.
It also makes it easy for themes and plugins to link to it using the
the_privacy_policy_link() function. The default themes all use this to add a link to the selected page to the footer.