The folder is/was an overarching concept which embodied how you would think about concepts in the real world — grouping together related items in order to achieve a task. The folder as an organizational paradigm worked really well because it was agnostic to what type of content you put in it, as long as it was a file compatible with the operating system. This was its most powerful feature — it allowed us to dump ideas and things which we thought belonged together, in one place, thus replicating our mental model as closely as possible.

For example, if you were writing a book, your folder would contain word documents, notes, pdfs, some presentations, all neatly organized into sub folders (of course). All of these file types were associated with their own programs (I haven’t used that word in a while), but could still be grouped together in a folder.