After putting the priming sugar in the keg after racking it why is it good to put a blanket of co2 while its conditioning?
It won’t matter as much since your naturally conditioning your beer. The main idea is to reduce aeration thus avoiding potential oxidation in force-carbonated beer, but again it’s not as important in your scenario because the yeast will more than likely use most of the available oxygen and replace it with carbon dioxide. ;)
Is there any particular reason your not force carbonating.
To answer your question…the reason for purging the keg with CO2 is to prevent further oxidation.