A developer uses the library in Python for web scraping purposes to extract data from HTML and XML documents. They parse the documents to create a parse tree from page source code that can be used to extract data in a hierarchical and readable manner. They are responsible for retrieving information by navigating, searching, and modifying the parse tree. They also handle tasks such as finding tags, navigating the parse tree, and searching the tree structure. This role requires a deep understanding of HTML/XML structures and proficient programming skills in Python.