I'll try to make an analogy using land property within a city:

The DoD had been holding millions of vacant lots in its ownership that were not for sale, nor use, and the addresses for those lots were not even listed in any property records. Furthermore, all roads leading to those vacant lots were blocked off to the public. So for all practical purposes those lots do not exist as far as the public is concerned.

Each lot has an individual unlisted [postal naming format] address which is basically that lots street name, street number, and zip code, and that address naming system is called "ipv4" [internet address format] to identify internet sites.

During the final minutes of Trumps term, the DoD suddenly, and without warning listed millions of those lots in the property registry, and opened the roads for the public to view the empty lots, and see what their addresses are. By doing so, the DoD very likely found people [hacker nerds] who had been squatting on those empty lots, and using those lots for activities that they were trying to keep hidden from the public.

We don't know what this move was really about, and we may never fully know. But the move is unprecedented, and considering the vast scale of "lots" discovered there is undoubtedly a purpose behind it.