Web hosting providers may or may not update your WordPress software for you (please check their policy), but all web hosts will not update your plugins.
WordPress plugins are created by multiple authors, and are consistently updated to newer versions to comply with the latest version of WordPress, or to address security issues and bugs. It is not guaranteed that the mandated updates to WordPress will not break the website, nor that plugins will continue to function if not updated.
For this reason, having a trained web agency on hand to monitor the website is necessary to update plugins, rollback plugins, replace plugins or even adjust the theme to continue optimal performance.