These "directories" can be mount points to other devices. For example, "/home" is frequently hosted on a separate disk partition like "/dev/hdb1". This is called mounting a device under a specified mount point. The mount point is the directory "/home" and the device is "/dev/hdb1". This can allow you to reinstall your entire operating system without overwriting your users' application settings. It also prevents your machine from crashing when some rogue user decides to download 150GB of mp3s of videos into his/her home directory and your machine runs out of swap space. On that same note, swap space is also a seperate partition with it's own "filesystem" or lack thereof.