If you have customized the package/software at all, either by editing the config files directly, or via a GUI, you may want to keep your customizations. Usually in Unix/Linux systems, configurations are saved in text files, even if the configuration/customization is done via the GUI.
Each Debian binary deb package has a list of files which it identifies as config files.
dpkg, and thus
apt honor this identification when removing packages, and also on upgrades. By default
apt/dpkg will not remove config files on package removal. You have to request a purge. On upgrade it will ask you to choose between the current version and the new version (if they differ) before overwriting config files. Even in that case, it saves a copy of the original file. Here Debian is trying to help you, based on the assumption that your config files may contain valuable information.
So, if you have not configured the package, or you don't want to keep your configurations, you can use
If you do keep the config files, then if/when you reinstall the package, Debian will attempt to reuse the saved configuration information. If the version of the package you are trying to (re)install has config files that conflict with the configuration files that are already installed, it will again ask you before overwriting, as it does on upgrade.
Minor comment: if you have removed the package and later want to remove the config files, you will need to call
dpkg directly, because
apt will not remove the config files if the package is no longer installed.
dpkg -P packagename
should remove the config files for you in that case.
From the man page of
I would tend to use