If the ADC requires a low impedance to take a sample quickly, then a source with high impedance cannot drive the ADC input.
For example if you monitor a 9V battery voltage with a 5V MCU such as AVR, and in order to not waste battery too much, you divide it with two 1 megaohm resistors down to half for the ADC input. It has a source impedance of 500k.
The AVR ADC input requires impedance less than 10k to work properly. The source impedance is too high to charge the ADC sampling capacitor quickly enough during the sampling phase and the sampling capacitor is not fully charged before the sampling phase is over and conversion of the sampled voltage starts.
Then you need a buffer op-amp, which has very high input impedance so it does not load the weak source, and can provide very low output impedance to ADC input that requires a strong source.
So the 200 ohms is not a problem, it is basically nothing compared to the required 10k, and sometimes it is even required.
Some op-amps are not stable with a capacitive load on output, and they don't work well when the empty sampling capacitor is periodically connected and disconnected directly to op-amp output for sampling.
In these cases the series resistance makes the op-amp stable when empty sampling capacitance is switched to op-amp output resistor.
And sometimes, like pictured, there is a simple RC filter before the ADC, which makes the AC impedance for the ADC input lower than the impedance of the resistor, and it may also function as simple anti-aliasing filter for the ADC input to prevent aliasing when sampled signal contains too high frequencies to sample the waveform properly.
Sometimes you don't need the op-amp, as you can also just connect the resistor divider or other high impedance source directly to the ADC input - and put a large enough capacitor on the ADC input to bring down AC impedance. This will make the voltage to stay relatively unchanged even when ADC takes a single sample of the voltage, but because after taking each sample, it takes long to charge the capacitor back to nominal voltage, it will limit the sampling rate how often the voltage can be sampled before the DC impedance starts to limit the sampling rate.
So if you need to sample a high impedance source at high sampling rate, the op-amp is necesary.