The information below seems misleading. I am confused with the example they give that if you lose dpkg (the program that lets you handle .deb files) you can use the other commands ar, tar, and gzip commands to download the .deb file for dpkg itself?
If this is true, what is so special about dpkg that is not available with the other commands?
As a Debian system administrator, you will routinely handle .deb packages, since they contain consistent functional units (applications, documentation, etc.), whose installation and maintenance they facilitate. It is therefore a good idea to know what they are and how to use them. This chapter describes the structure and contents of “binary” and “source” packages. The former are .deb files, directly usable by dpkg, while the latter contain the source code, as well as instructions for building binary packages.
5.1. Structure of a Binary Package The Debian package format is designed so that its content may be extracted on any Unix system that has the classic commands ar, tar, and gzip (sometimes xz or bzip2). This seemingly trivial property is important for portability and disaster recovery. Imagine, for example, that you mistakenly deleted the dpkg program, and that you could thus no longer install Debian packages. dpkg being a Debian package itself, it would seem your system would be done for... Fortunately, you know the format of a package and can therefore download the .deb file of the dpkg package and install it manually (see the “TOOLS” sidebar). If by some misfortune one or more of the programs ar, tar or gzip/xz/bzip2 have disappeared, you will only need to copy the missing program from another system (since each of these operates in a completely autonomous manner, without dependencies, a simple copy will suffice).