You can regard the capsule as a pure capacitance of say 50pF. The generator is different, having a finite output impedance. When you couple this to the DUT (Device Under Test, K.H.) with a large capacitance, then you are driving the amplifier from a low impedance completely unlike the capsule.
This means that amplifier noise will be much lower in the DUT. So this may have effects on noise and distortion measurements. As an advantage, the normally very high input impedance is swamped by the generator's low impedance, meaning noise figures will be very good. Remember the normal amplifier (the condenser mic's impedance converter, K.H.) has an input impedance in the range of 500 megohms to several gig ohms, depending on design.