A C/C++ STL (Standard Template Library) developer uses the STL library in C++ to develop efficient and reusable software. They write, test, and debug code using STL's data structures and algorithms, like vectors, lists, queues, and sort or search algorithms. They use STL's modular components to enhance productivity, reduce coding errors, and make code more maintainable. They also optimize code for performance and memory usage. Additionally, they may contribute to the development and enhancement of STL itself, fixing bugs, improving existing components, or adding new features, thereby enhancing the overall efficiency of C++ programming.