One of the main reasons I got out of the military was when I got promoted high enough to go to "staff meetings". Higher staff would sit around and plan activities for the troops. I was dumb enough on some of the ideas to ask "why?". 90% of the time the answer was "we can't have to troops just sitting around doing nothing".

Maybe a front for a dumb (deep underground military base) or nuclear shelter...possibly deep state escape hatch to underground rail network. The architecture suggests the building was built to prevent extreme attack, with small, narrow, windows and heavy support in between.