When the system is installed using CDROMs, the Packages file is read from each CD inserted during the installation process. These files are used to update the apt database so that apt-get will be able to find the packages it is asked to install. For the 2.2 release this amounted to 4 binary CDs when the complete archives is included, providing over 4000 packages.
Under these circumstances you will not need to update the apt-get database again until you are ready to use a new release set of CDs. Then you will use the command apt-get update, and insert the new CDs when requested. This will bring the database up-to-date with the packages on the new CDs.
If you use an FTP archives of Debian to maintain your new system in the future, you only need to do an update whenever the archives changes. For the stable release this is quite rare, although it does happen, making it useful to keep track of the latest update release for stable. Running update when the release changes will keep your system up-to-date with the archives.
If apt-get's sources.list file references the unstable distribution, you will probably need to run update each time you wish to upgrade your system. Since the unstable distribution changes often, and without notice, this is the only way to keep in sync with the archives.