I have always wondered the same thing. I can't find any documentation to support it, but typically this pattern is used to make it easier to store files on another volume (e.g. hard drive). This allows the drive to be mounted (e.g. attached) into the file system in one place. For example when the drive is mounted at /private and then the folders /etc, /tmp, and /var are then all located on that other drive.
What I can't say is why this would be beneficial. It is worth noting, however, that these three folders contain "data" files such as configuration, temporary, log, transient and database files rather than executable code contained in the /bin, /sbin and /usr folders.
