For winter camping warmth. This inspired idea comes from the amazing Tim Tinker, a source of so many wonderful winter warmth ideas, particularly ultralight tent stoves. See for example Mostly Perspiration. You can read all about the idea here. The entire set-up weighs around 67 grams and keeps your sleeping bag toasty warm (in the snow!) for up to 12 hours! Genius!
Mind you, you probably already carry a soft water bottle. This 2 litre one was just a cheap Amazon version of the legendary Platypus weighing 33 grams. You can also probably find some mylar coated bubble wrap amongst your possessions somewhere. This is from a windscreen protector. It added a further 34 grams.