Well, for a start, and at the risk of stating the obvious, automated testing is faster. And with the arrival of continuous integration, continuous delivery and continuous deployment, automated testing is even more of a no brainer, because it results in faster turnarounds and the faster identification of defects.
Another benefit is that automation invariably improves the testing and development teams’ relationship. There can often be a little dissonance between test teams and developers, but the nature of agile and DevOps, means they have to work better together. Sharing automated scripts means that tests can be executed and any issues identified before passing over to Test. These tests run automatically as soon as new source code has been added informing both teams if they fail. This bridges the gap between testers and developers as they work more closely together.
It is also true that test automation improves overall software quality, as it can be utilised earlier on in a project to do this. And it also increases the depth and breadth of the test suite – test automation can increase the overall test coverage and can execute tests that target part of an application, which would otherwise be impossible to test manually. Provided the implementation of automation tests have been done correctly and are maintainable, they are often reusable and can be executed on different versions and platforms.