Everyone tests their code

No one writes code, commits it and pushes it to production without checking it works.

Everyone tests their code, but they usually do it manually. They switch from the code to a browser or terminal, run the code and evaluate the result.

This takes time, you need to switch contexts, and the test is only valid for that time.

There's no guarantee it will still work in the future.

Automated testing, however, means you can write your code and run the tests without leaving your IDE or text editor.

The tests can also be run in the future to ensure the functionality works without needing to re-test it manually.