I work in engineering, not automotive but aerospace. In my particular area we deal a lot with graphite composites.
The main advantages of composite over steel is that you can fine tune composite for the specfic load charataristics. You can drastically increase torsonal stiffness by altering weave angles, combining weave angles or adding more layers. You can do this with steel or aluminum but at the cost of weight.
The other advantage of composite is that it can handle many more cycles than metal. Over time metal will fail, when constantly flexed, before composite.