Dozy
In case of an AoA failed in such a way that erroneously gives a stall output, the normal law would pitch down to reduece the AoA. Because it would remain fixed, erroneously, FCS would keep pitching down... Or so I believe.
The fbw can take over and we need to be able, firstly, to detect that normal law is no longer desirable and secondly to revert to a safe control law.