How many times have you deployed a new release in a project, and only after merging it to production an urgent bug is detected? This usually happens for lack of testing, for following an incomplete test guide, or for human error.
For continuous delivery projects, where you are delivering new code constantly to your users, automated testing is critical to make the whole process agile. Not just to maintain the quality of the code-base but most importantly to ensure the stability of the web platform throughout its development cycles.
Even though this is well-known in the web development community, it is not always easy to have automated tests as a priority and to convince decision-makers to dedicate time and resources to implement these tests.