Just like a Military DMZ and IT DMZ Serves the same purpose. To keep enemies at bay and provide a protective buffer between the hostile world of the internet and the internal corporate environment.
While in Practice Both Military and IT DMZ can be thwarted. As in the Military and IT you can just go "around" the DMZ. In the real world this is physically going over, under, or around the DMZ.
In IT this is doing the same thing going "around" by social engineering or phishing a user, or getting users to install weak or unapproved software with backdoors built in. Think SolarWinds, Zoom, etc..
Sometimes the can just go through the DMZ because Businesses don't implement them correctly.
But what is an IT DMZ? well the definition is.
A segment or subnet of a private network where resources are hosted and accessed by the general public from the Internet. The DMZ is isolated from the private network using a firewall and is protected from obvious abuses and attacks from the Internet using a firewall. A DMZ can be deployed in two main configurations. One method is the screened subnet configuration, which has the structure of I-F-DMZ-F-LAN (i.e. internet, then firewall, then the DMZ, then another firewall, then the private LAN). A second method is the multi-homed firewall configuration, which has the structure of a single firewall with three interfaces, one connecting to the Internet, a second to the DMZ, and a third to the private LAN
In Information Technology practice it is not longer a Physical concept, It is completely virtual thus further eroding its effectiveness.
Read more at Woods LLP