The poncho liners is a light weight polyester fill camo blanket designed to tie into a military style poncho so you can use the poncho as a sleeping bag. It can also be used alone in warm weather or used inside a real sleeping/bivvy bag when colder.

Aka - woobie, binkie or blankie (when kidding around)

This remarkable piece of gear is very light, fairly warm, quick to dry, and has various uses. It can be folded fairly small, but normally is just crammed in and around other gear.

There are civilian knockoffs - some better, some not so good.

Regardless, this is the one piece of kit that went into the ruck regardless of the terrain/environment/mission.

I do agree tho, the P38 can opener has it's own place in the military gear that really works hall of fame.