Authentication Local plugin used by the Mobile app to check:
- Web Services enabled
- Mobile Service enabled
- Site is in maintenance mode
- The user has to log in in the site using the browser (instead in the app)
The app doesn't need this plugin installed in the Moodle site in order to work. But if your site uses SSO auth plugins like CAS or Shibboleth is mandatory to install this plugin and configure it to force the user to log in using the browser (in the Moodle site).