Good point. I tried to leave it as I found it originally. Now, my understanding about the back flow preventer was in case you were connecting the hose to one of those fertilizer or weed killer spraying attachments and to prevent contaminating your water supply.
There was an anti-siphon attachment on the faucet previously (which probably had something to do with the stripped threads). Looked almost exactly like this.
Otherwise we always winterize the outdoor water supplies by shutting of the water to them and letting them drain out.