Purpose of ADuCM360 ADC buffers

I am having trouble understanding the function of the optional buffers between the PGA and ADC on the ADuCM360. It seems the buffers cannot be used at all for Gain=1 when the PGA is off due to a silicon error so I think that the ADC sees the MUX output completely buffered. For PGA gain >2 you can enable the buffer but doesn't the PGA already serve as a buffer between the input and the ADC? What purpose does adding the additional buffer serve. Is the PGA output impedance too high to drive the ADC? It is not clear from the datasheet and manual when you would want to use this feature which increases power consumption.