To one that does not want to rule and conquer the world, spy agency is a way to control the information of the world, to sell it to the highest bidder, to assist capable rulers.
But for someone like Aeryon who wanted to rule, the spy agency is necessary to supplement the army.
If Aeryon has information but he could not use that information because of his soldiers is insufficient, it would feel like there is a delicious food in front of him but he could not eat it
That is why before, he focused on the army building, rectifying the discipline and fighting the war the way the tribes of the Steppe fight the war.
But once the army is stable, morale is high, the discipline is there, Aeryon focused on creating a spy agency.
Aeryon did give some of his insight on how to create an espionage agency to Mayeux
