When you download a Debian "netinst" ISO and use it to construct a system, you reach an installer screen with the title "Software selection". By default, that screen enables the desktop environment and offers you a choice of windowing interfaces.
If you leave the desktop environment enabled then both Network Manager and the Avahi daemon (multicast DNS) are installed and configured, and the system will boot into the desktop windowing environment.
However, if you disable the desktop environment in the "Software selection" screen then the system will boot to the command-line console and neither Network Manager nor the Avahi daemon will have been installed.
It is not immediately clear why electing not to install a windowing environment should also affect data-communications features. It just does.