Why does the back-end guy need to ... what? What kind of spaghetti does this guy and his colleagues build where there is such strong back-end and front-end int

Why does the back-end guy need to ... what? What kind of spaghetti does this guy and his colleagues build where there is such strong back-end and front-end interdependency that the server-side code cares what the front-end is doing for DOM manipulation or client-side-MV* like this, I wonder.