An extremely robust and lightweight alternative to plastic or metal bottles. When empty, these light but tough water bags may be rolled up, and take up hardly any space in your bag in contrast to bulky containers or bottles.
Thanks to the strong straps they are comfortable to carry, can be rolled or hung up to save space e.g. in the tent or on a tree. They make great pillows for sitting or sleeping, can be used as hot-water bottles, and have even been known to function as water ballast on para-gliders.