Understand hidden logic of application. It often happens that testers receive very poor requirements or do not get requirements at all. It also happens that analyzing the code is the only way to understand the logic of application without having any sort of requirements. Such analysis helps to approach test design more effectively (in aspect of test data preparation) and in general to detect risky areas of application. It will also help you to assist your developers in issue root cause detection and hence to grow your professional reputation.
Use advanced techniques of test automation. Some types of user-to-web page interactions are impossible to implement with the help of only bindings for chosen programming language (for example screen scrolling, creating custom elements within the page on test run time, DOM analysis, etc.). Disclaimer: You should use advanced techniques of test automation with caution and always have a good understanding of what you’re doing and why. Otherwise there is a high probability of missing important defects.