postfix) may install or generate configuration files (files that usually go in the
/etc folder). During this process, there may be options that can be changed by the user. On installation, you are usually asked to set these options. In the case of
postfix, it involves what type of server you're running, the hostname, and allowed IP addresses (among other things).
If you wanted to change those options afterwards, though, you can use
dpkg-reconfigure which only goes through the configuration files phase of the package, where you can change those options.
Note that not all packages have user-configurable options, so even if a package installs a configuration file in
dpkg-reconfigure might not do anything.