The use of plastic bags

In a shop when you pay they will always ask you:" Pose??"(puse). That means plastic bag in norvegian. They are you useful for a lot of things... the normal way to carry stuff, as a garbage bag, as a slide in the snow,etc. here you can often see that plastic bags are on saddles of bikes. i asked my self what do they do there. the answer was keeping the saddle dry, or keep the humidity inside the saddle.... either way is possible. it depends on if you remember to cover the saddle before it rains, or if you forget it....