A Composer developer primarily manages dependencies for PHP projects. They specify libraries or packages a project relies on, and Composer automatically manages installing or updating these dependencies. This includes ensuring the correct versions are used to maintain compatibility across all elements of the project. They also ensure the autoloading of these dependencies, making them available to PHP scripts. Furthermore, they may maintain Composer.json and Composer.lock files, which contain metadata about the project's dependencies. Composer developers can also create and distribute their own packages through Packagist, the main repository for sharing PHP packages that Composer uses.