Automated testing offers repeatability

You work on feature or bug fix.

It gets tested manually by you, a tester, and the person who requested the feature or reported the bug.

It may get tested on multiple environments.

It passes and it moved to production.

But now it's live, it's not tested again.

Although it works now, there's no guarantee it's not broken or regressed by subsequent changes.

It's definitely not tested manually before every future release.

Here's the thing

Automated testing offers repeatability.

The tests pass, so the functionality works when you wrote it, and you can re-check it by re-running the test.

The test suite can be run by Developers whilst working on other changes, before code review and before deployments.

The test suite can be run automatically in a CI pipeline for every change that's pushed, and you'll know your change not only worked at the time but will continue to work.