One of the first things one learns as a quality assurance specialist is that there are three basic types of automated tests: unit, integration, and end-to-end tests.
Almost every software development agency eventually creates a QA department. QAs' play role is to validate if the product meets both business and technical requirements, but also works and looks as expected. Yes, a company can do just fine with no testing experts, and make their developers take care of that part. However, if you crave effective, automated testing process, you need field pros within your team.
People make mistakes. No matter how accurate we are, no matter how much we try, when working on modern, complex applications misunderstandings and misconceptions are embedded into our daily work. The key to success is to make sure that we have skills and processes in place to detect and eliminate those as soon as possible. This is where the QA team comes in. Not only does the team care about catching and describing bugs, but also about making sure that all new features and stories are described in an understandable and meaningful way. This helps the project move forward smoothly, keeping both team and the customer happy and confident.
When you're an experienced front-end developer, it takes just one look and few mouse clicks to judge whether or not the site in your browser is correctly implemented. By working on these problems day after day, you'll write dozens of lines of HTML/CSS, checking the outcome each time.