Well, in this case “folder” could be substituted with a generic “item”, I guess. We still have items: songs, projects, videos. And in order to let computer know which items we want, we somehow have to organize them. This is where experimentation happens in UI, but it's all based on “moving” items between folders (playlists, etc.). It's familiar to us from real world, which makes it convenient and easy to learn.
There are, though, UIs where item organization may happen without user input, like Genius playlists in Apple iTunes. This might be the future, I guess.