Obsolete, all skilled Deity players (you aren't one. To be skilled, you must win at least 95% of the time, without regenerating.) know that at 2048 the AI literally floods you with aircraft, trying to prevent you from winning a time victory. If you don't watch out, all your cities will be taken by Commando paratroopers the next turn. Bunkers, however, do an excellent job in preventing this.
You can sometimes rushbuy a bunker in the heat of the battle, but aside from that it doesn't have a lot of use indeed.