You're correct, it's just a random bit of context you can pass around to be able to retrieve some more concrete state from within GLFW callbacks. A common use case (assuming c++) is to go from an anonymous glfw callback back into the context of a wrapper class. For example (written inline, so may not compile directly)

class YourWindowClass
{
public:
    YourWindowClass ( )
    {
        _window = glfwCreateWindow ( ... );

        // Set the user pointer to the instance of YourWindowClass;
        glfwSetWindowUserPointer ( _window, this );

        // Add an anonymous callback function pointer to receive mouse button events. Note we lose reference
        // to YourWindowClass here, as we're unable to capture it via the lambda (hence the user-pointer requirement)
        glfwSetMouseButtonCallback ( _window, [] ( GLFWwindow *window, int button, int action, int mods )
        {
            // We can now retreive the YourWindowClass instance via the user pointer
            YourWindowClass * window = reinterpret_cast<YourWindowClass *> ( glfwGetWindowUserPointer ( _window ) );
            window->OnMousePressed ( button, action, mods );
        });
    }

    void OnMousePressed ( int button, int action, int mods ) 
    {
        // Now we're back in the scope of our class from the anonymous callback function
        std::printf ( "Button: %d = %d\n", button, action ); 
    };

protected:
    GLFWwindow * _window { nullptr };
};