Basically, I can afford to build two combat planes this turn only because I got the extra pay for damaging the Kaiju. Since I was the only one to destroy a building, I don't expect to keep a high income. However, if I have reason to believe I'll have customers who can offset the cost of producing planes, I can build more workshops and have a higher plane output, without needing to worry about fielding them and doing well *myself* before investing in infrastructure.
As for why planes are good, they do more damage than anything else we can field (note how much the fighters did in the last combat) while requiring effectively net zero hangar space. Since they fly to battle, and other units need a transport plane which has the same size as a fighter, they're cheap to store and reuse. And, hell, even if people don't want to deploy fighters, they still need troop carriers to carry their crab tanks and infantry around the world. I can provide higher capacity transports at lower cost, using the same facilities as needed for fighters.
Oh, also, I can give you kaiju bits. I don't have a dissection lab, and don't have spare scientists, so even a fairly poor trade is worth it for me.