As I understand it, the pre-boarding is for uniformed military personnel, which I take to meaning traveling on orders. I saw several cases of military heading to or returning from deployment (Afghanistan/Iran) who had pre-boarding and were applauded onto the aircraft. While there aren’t so many of those these days and the term may have become sort of redundant, imagine the outcry if UA or any airline rescinded the benefit.

Btw, in a few cases (very few) I’ve seen the captain go back into Economy and invite and accompany a uniformed military to an open seat in J or F on TATL. The flight attendants asked us (when seated in F) if we had any objection. On deplaning at LHR all four of us regular F pax (763) thanked him for his service. This guy was very young.

I do wonder if the benefit extends to overseas uniformed military who happen to be flying UA (say Canadian or British).