Why is a Buffer+RC-combination recommended for driving a SAR ADC analog input?
Someone else will most likely pop in with a better answer but as far as I know, the RC circuit has two components.

1) the SAR needs to see a stiff input when it samples the input.  That could be either a capacitor or an op-amp that is stable with a heavy capacitive load.

2) most o-amps are NOT stable with heavy capacitive loads.  Especially fast op-amps.

So the R isolates the op-amp from the heavy capacitive load presented by the SAR.  But now the input to the SAR will sag because of the resistance.

The capacitor is added so that the input signal seen by the SAR doesn't sag very much when the signal is sampled.

In other words, even though it looks like a RC low-pass filter, that is not the intended result.  Just consider it to be a freebie bonus!