You can also put your bathing suits, beach towels, etc in them. They flatten out to almost nothing.
If you have kids going with you, they are perfect for extra t-shirts, socks, pj's. On our cruise with our grandchildren, my daughter just got a couple of packages of men's small t-shirts for the two children. They wore them to bed, going back and forth to the pool, great to put on when you're at the beach and the sun gets too hot and they need to cover up. You wll be surprised on how much stuff you can compress in one of those bags, they really save space in your luggage.