For large code bases in dynamic languages I think the advantage grows larger over time. Even a modest amount of test coverage will allow you to build new features without breaking existing features.
It's a lot faster to catch bugs when the tests run in your local development environment or in the CI pipeline than to wait until Q/A (hopefully) catches them in end-to-end integration testing and sends it back to you for rework.
The other advantage I've noticed is that the tests can serve as an additional form of documentation. New team members can look through the tests to see how code is supposed to work from the examples there. For some web API projects I've even been able to generate the documentation from the test cases.
It's a lot faster to catch bugs when the tests run in your local development environment or in the CI pipeline than to wait until Q/A (hopefully) catches them in end-to-end integration testing and sends it back to you for rework.
The other advantage I've noticed is that the tests can serve as an additional form of documentation. New team members can look through the tests to see how code is supposed to work from the examples there. For some web API projects I've even been able to generate the documentation from the test cases.