It is a matter of checking - there are 3 Flight Control Computors (FCC's).
Each FCC can deliver its output in the form of Flight Director (FD) guidance, or in the form of AutoPilot (AP) commands.
Normally, Left FCC supplies Left FD, Right FCC supplies Right FD and now if you use Centre FCC to fly the a/c (through Centre A/P), you have a constant check that all FCC's agree.