The Air Defense has its sole purpose in defending a base vs airborne threats by combining high Range, Attack power and Armor.
The Air Defense is first unlocked at HQ2 and when excluding Vertex, which is technically not an anti-air defense, the strongest defense against airborne targets the confederation has to offer. Combined with its relatively high HP and Armor, this means, that it can easily win a fight vs fighter jets in equal numbers. It also becomes more effective vs jets when they appear in higher numbers, as fighter jets tend to overlap midair when not microed properly, enabling the Air-defenses' rockets to damage multiple of them at the same time.
In general its best to place them in between other (defensive) buildings, as this keeps enemies from completely bypassing the Air-Defense by going around it. This also has the added advantage of protecting them from ground attacks as units tend to first focus on the closest building if not ordered to attack a specific target. It comes at the disadvantage though, that this makes ones' base more vulnerable to bombing attacks as their high splash damage area allow them to then damage both the Air-Defense and also the buildings right besides it. It is for this very same reason, why 2 Air Defenses should never be placed in the neighboring cells from each other to keep them from being hit in one attack, because fighter jets, even if it is smaller, also have a splash damage area.
Last but not least, they can also be used to fight against Dragonfly rushes, as they are capable of quickly killing them, while barely taking any damage from their machine gun and only a little from their rockets.