The anti-air position is a sand-bagged fortification armed with twin Bofors 40mm flak cannons. The position saturates the air with flak when enemy aircraft come within range, thus eliminating early-game aerial threats.
Make sure to place the anti-air bunkers near key locations that the enemy will be tempted to bomb (like your airfields). Though the flak guns make short work of unarmored vehicles and infantry; this bunker has very little defense against armored units; be sure to supplement the AA Bunker with anti-tank bunkers.
Bunkers are necessary for a good defense, as units themselves cannot defend a base against a combined attack. With the anti-air bunker, America enjoys the first anti-air weapons in the game.
The anti-air bunkers take up quite a bit of important room in bases, thus, many players prefer mobile AA or hiding large groups of light AA in woods. However, experience shows that a few of these dotted around your base can hold off even major bomber assaults, provided that their target aren't your bunkers.
In general, the AA bunker is vital to protect your base from total annihilation by an enemy bomber assault.
Med. Twin AA Gun