Backflow removal necessary?

First of all, thanks for your input on my last concern.
I am wondering, why do you have to remove the backflow preventor if you have already blown out the system and there is no longer water to freeze?
Thanks for your input.
Nereus

Some will say that it isn't necessary to pull the BF out, just blow it out, but it's too easy not to pull it to take a chance IMHO. Any water that remained in it would freeze and break the device so I just store mine where it won't freeze.

An RPZ is made of brass, a soft metal. You don't want to take chances. Plus, you cant blow past the RPZ into the house, so that line coming out of the house would have water in it.