Not all Debian systems have a GUI, and even though using WiFi on a server isn't common, there are plenty of instances where you're using WiFi with a headless setup, like on a Raspberry Pi. It's not difficult to connect using only the tools provided out of the box in Debian.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-connect-to-wifi-from-the-cli-on-debian-10-buster