Think about the collaboration and teamwork taking place in large, distributed teams of developers in different countries, and imagine they want to collaborate on the same source code repository and check code changes at different times. It's very important for these teams to understand the code and be able to test it locally to see how it works, whether their changes will impact existing services, and whether the code is still working as expected.

Testing is important to ensure we have quality in the delivery or user experience. By having lots of tests, you can identify defects early and fix them. For example, if there are major bugs detected, you could decide not to release the recent update and fix the issues before doing the release.

Another major point is usability. For example, your client might have performance or non-functional requirements. Imagine an e-commerce website, for example, where you could add items and would have to wait a whole minute for them to be added to the basket. In most cases, that would be unacceptable and the user would lose trust in the platform. Ideally, you would have a testing process to make sure that the latency is still low and the website is responsive. Other examples that require testing would be features that are not working as expected or user interface defects that prevent the users from completing the tasks they want to do.

It is important to have shorter release cycles. Using automation, you can run a thousand tests automatically and consistently, without needing a human to test different parts of the site manually, test the APIs manually, or rigorously inspect the code before any release. Before every release into production, you run those thousand tests, making you much more confident that everything is working as expected, and if you do spot an issue those thousand tests missed in production, you can fix it and add a new test for that scenario too.