The Air Field provides 4 slots required to build and refuel Vertex and Thor in between attacks. Furthermore it's the source of their passive repair.

When building multiple Airfields, there are several ways to place them efficiently:

Close together:

This allows for best micro-management of Vertex as they start close together and therefore don't have to be aligned before usage. It also keeps Thors in a straight line when bombing, which minimizes the time Anti-Air weaponry can fire at them. This setup is best when playing aggressive, but there is a high risk of loosing all avia at once if the enemy manages to break into the base at this point, being the most extreme when superweapons are used as they can easily destroy 9 or more tightly placed airfields.

Scattered:

This is the best setup when playing defensively, as it makes it more difficult to target all airfields with a superweapon and if your enemy has a superweapon, your only chance to destroy it before it fires again is with avia. This means that when the enemy deals a devastating blow to ones base there is still a chance to strike back.

Furthermore, independent which buildstyle is chosen, it is always recommended to build at least 1 Air Defense near each Airfield as Vertexes without fuel and Thors are vulnerable from Hawk attacks. It is not recommended to defend them with Typhoons though, as they deal roughly 20% less damage than Air Defenses while also taking away valuable Command Points which could instead be used for more avia units.