The Mylar emergency blanket, also known as a "Space Blanket", is an extremely versatile item. Rather than insulate, they reflect heat. This heat reflection makes them useful as a shelter, blanket, or protection from the sun. They can also be used to keep items such as hot food warm or to build a solar oven. It should be mentioned, however, that they are rather fragile and you should always be gentle when using them. Carrying more than one adds little weight, and will provide you with a backup should you tear the original to the point that it is no longer useful.
In addition to the standard space blanket there are also mylar emergency bags aka space bags which are superior to a simple blanket in that they provide a barrier between the ground and the user where the blanket alone does not.