For maintaining the performance of the ADCs even at high frequencies, it is important to make sure that the ADCs are able to draw power from their inputs.
The internal capacitors of the sample and hold circuit of the ADCs have to charge as fast as possible. In conclusion ADCs should be connected to a low impedance signal source. Due to the added impedance of the previous stage, the impedance has to be converted by an impedance converter or also called voltage follower.
This can be done by using an op-amp and connecting the output of the op-amp directly to the negative input terminal. This leads to a very low impedance input for the connected ADC.