They are used for such items as t-shirts, underwear, anything that will "fold flat" in your luggage. The main reason to use them is that they take up less space in your luggage. Just put the items in them, roll them up to get the air out , seal them shut and flatten them out. They are a lot less expensive than the plastic ones that you have to vacuum out the air.
Systems such as pack-it folders sold by Magellan's and others are also good for shirts, etc.
Like I mentioned earlier, they take up less room in your luggage.