You can have a pure designer who doesn't understand code, but can follow the spec for interfacing with the frontend devs (give them a PSD). Just as those frontend devs might not understand how to best design a database-driven backend, but they can follow the spec for interfacing with it (REST calls through xhr). And those backend devs don't have to know about frontend application frameworks, they can follow the spec for interfacing (REST endpoints).
You can have a pure designer who doesn't understand code, but can follow the spec for interfacing with the frontend devs (give them a PSD). Just as those frontend devs might not understand how to best design a database-driven backend, but they can follow the spec for interfacing with it (REST calls through xhr). And those backend devs don't have to know about frontend application frameworks, they can follow the spec for interfacing (REST endpoints).
Again, modularity and separation of concerns.