Is a buffer needed to measure the output of a power supply using an ADC?
The buffer is used to convert the impedance from the source into a low value so the input impedance of the ADC itself does not significantly affect the voltage measured.

Since you are going to divide the voltage down to something usable by the micro via a voltage divider, and since you will want to use large resistors for that divider to limit the power losses, you end up with a high impedance source. As such you need to buffer it after the division before presenting the value to the ADC.