Packability
They hardly take up any space and will make up for the weight in usefulness
Best Use
Internal pack organization, drying items, keeping things stink-free
What this bag could hold
4 liter: some snacks and drinks or extra layers
6.5 liter: a layer and a meal or so worth of food
9 liter: a puffy jacket & a sleeping bag or food for 2-3 days
15 liter: all your clothes for a trip
20 liter: a tent or any Sea to Summit sleeping bag