What are folders for?

Folders actually do have one function - they have a weight of 0 but can be sold for 1 cap each, so there's no drawback to picking them up other than the time spent doing so.

They have no known use. They can't even be scrapped for materials.

Arguably they could be used for "decorating," or collected to see how many you find (and so on), but serve no in-game function - the same as burnt magazines, burnt books and some other miscellaneous items I can't think of off the top of my head.

Because of their lack of use anywhere else, and their lack of weight as well, they are the perfect ammunition for the Junk Jet, alongside the Pre-war cash stacks