I've been waiting to type this for a while...
When you blow out the lines, water is still left in them (water clings to the pipe surfaces and will, eventually find it's lowest point and puddle there). If one does a 1/2 ***ed job of it, there is enough to fill the pipe it's in and, if the water freezes - you could burst the pipe (I seem to recall a discussion on 'lateral expansion' of ice here). The antifreeze should mix with whatever water is left in the line and provide enough protection to prevent the water from freezing and compromising the pipe. For many pool owners, who had the job done by a competant builder, it should never be a problem because the pipes should be run below the frost line, however Ma Nature can always decide to have a record breaking season.

Richard, I don't think that anyone is talking about adding 1000s of gal of propoline glycol to their pool water to prevent freezing, just to the water that will remain in the pipes... but you never know