an FBO can use the color, depth and/or stencil buffer of the FBO 0, i.e. one can attach to an arbitrary FBO the system provided color buffer as a color buffer and along those lines the depth and stencil buffers. One use case is to simulate multiple depth and stencil buffers. I suspect that it might be that one will not be able to attach the depth buffer from the system but a stencil buffer from elsewhere to the same FBO, and visa-versa.
One can simulate this by making one's own stencil, depth and color buffers and just carry on as if that is what to render and at the end of the frame to blit the color buffer to the window color buffer; but that seems silly and sounds like a hack to get around an API issue; Moreover, we will see OpenGL (not just OpenGL ES) on embedded systems where bandwidth is tight and the resolutions are insane, so I think this is worthwhile API to add.