Originally Posted by casebrew
Then, when I transfer to a secondary, the beer has about 0ne volume of CO2 in it, to come out and sit in a layer on the beer, protecting it from oxygen, and displacing the light oxygenating air out the top.
That's true, the problem is that while you're racking the carboy is full of O2 and any splashing at this point needs to be avoided. If you purge your carboy, there is no O2 in there and splashing a bit while racking is less of an issue. Oxidizing has the biggest affect on the stability of the finished product. So while one may not care about a session beer which is going to be gone in a month, the CO2 purge is probably a good idea for beers which require long aging like barleywines and imperial style beers. I've also read that it is essential for making meads since they hang around in carboys for a year at a time, and need to age much more than that.