Allies help make your city stronger and can also assist you with earning more resources! Core principles to know…

#1 Allies hired (owned) can provide you with extra bonus when you are attacking, defending.

#2 Allies also provide you with a quick way to earn gold! Any time an ally is hired away from you, you will get back what you paid for the ally and a little more gold on top of that! Two or more players hiring an ally back and forth between themselves is called "Volleying"! 

#3 Allies give you a Resource Bonus. Meaning you can earn more resources through the marches you send out against PvE objects in the game.