The idea is to automate repetitive tasks as much as you can because doing them by hand takes time and is error-prone. Build, deployment and testing are tasks that are well suited to automation.
I started by automating things that would take too long. At first it was taking code from my repository and deploying it to Heroku. Then I discovered that CI services literally gave you servers to run scripts on to interact with your code after a git push, so I started running tests in these servers.
Fast forward a few years and I automated everything except writing and maintaining the codebase.
I even run linting tests in CI so no badly written code can even make it to code review.
