Very simple and very useful.

I have a few different bags packed including a (few) disposable supermarket bags with handles. Useful for all sorts of things,
including carrying wet or dirty things outside your pack and
good to collect stuff in.

For example, we "issue" them to the kids when on a beach walk, so they have somewhere to stash their loot - by the way, if you issue a small bag, it does limit how much they can collect !