For RNW Media I recently set up a CI/CD system to run on all Drupal 8 sites (and prepare for Drupal 9 and other frameworks). Continuous Integration allows a team of developers to quickly release and deploy on their Staging and Production environments using only version control (Git). For a dev team this is a massive win. It means you can deploy as frequently as you like without a major hassle on a live site. It also reduces the risks of down time due to less manual tasks, and less chance on human error.
Having an automated deployments does require a fully functioning build system. To accommodate the large amount of custom modules, themes and generic plugins, I also set up a private packagist. Now all custom code is distributed as composer packages. This makes for a more maintainable code base in the long run, better reuse of existing components, clear versioning and more.