(a) CG and MAC are the same thing looked at in a slightly different way.
CG is the distance of the Centre of Mass from a datum position nominated for the measurement of fore and aft distances (arms).
(percentage of) MAC is the distance of the Centre of Mass from the leading edge of the (theoretical) MAC (mean aerodynamic chord) expressed as a percentage of the length of the MAC.
(b) if the CG (or %MAC) is entered into the sums, the FMC may be able better to optimise some calculations for which CG is relevant.