.dmg files organization Today I needed to view the contents of OS X's SDKs, which are published as .dmg's.

To my surprise, I discovered that the structure is as follows :


The .dmg contains a hidden .pkg files
The .pkg files contain a Payload file, which is a .gz or a .bz2, depends
The .gz contains a single cpio-compressed file
This cpio archive contains all the files.


Can anybody provide technical and/or historical reasons why this is done this way ?

Just to be clear : I don't have any technical problem, I'm just curious.
The .dmg is the disk image which is mountable like an external file. The value it provides here is that .dmgs are checked for consistency/correctness upon mount, so if the download went wrong you will get notified
The .pkg is the standard format for installable software and usually contains not only the application itself but all the other stuff displayed during installation: ReadMe, legal/license information, optional packages etc.
The .gz is to compress the data and keep the packages smaller on disk (and for download)
.cpio is an archive format similar to tar etc.