The reason is that the possibilities are used to test a conception of how things are. Here they are being used to test the conception of mind and matter that gives rise to them. The argument is that if mind and matter are thought of in the Cartesian way, then there would be wide-open possibilities of a bizarre kind, about which we could know nothing. So, since this is intolerable, we should rethink the conception of how things are (this is called the metaphysics).
This is essentially what i noted above, that no one means to acgualy entedrtain such possibilities
