Nothing special.

Good as extra-insurance against a saturated pack. I always pack my clothes inside a trash bag and sometimes my sleeping bag. A wet down bag is as useful as a glass hammer.

These also come in useful when hauling your food into trees, you can poke a hole in the bottom and have the bag cover your haul-pack, incase it rains in the night. No sense in getting your choclate bars all soggy now is there.