A JSTL (JavaServer Pages Standard Tag Library) developer designs and builds web applications using JSTL tags in JSPs. They use JSTL to encapsulate core functionalities, like iteration, conditionals, and XML manipulation, making it easier to read and maintain the code. They also manage data processing and storage through SQL tags, and format and localize data using formatting tags. A JSTL developer helps in making the application logic independent from the presentation layer, enhancing the application's scalability and flexibility. They collaborate with other developers, continually test and debug their code, and keep up-to-date with the latest industry trends and technologies.