Even a durable plastic shower curtain liner needs to be replaced every once in a while. When you do, launder your old one, and save it for your next picnic or outdoor event. By laying the old waterproof liner on the ground and placing your blanket on top, you're helping to protect your blanket and create a waterproof environment to rest on. Even a little bit of dampness on the ground can build up over time, turning your evening in the park into a wet and soggy mess. Plus, you don't have to launder your blanket when you get home.