Usually after you outgrow the regional foe (mystmountain bandits, raiders, snake parties) It's usually time for you to either take on smaller jatu patrols (~40-90), roaming knight companies (~20-50), or start taking on caravans and lords.
After the beginning 'phase' of just getting a foothold you usually need to change you army composition to cater to the enemy you want to face. There is a great section in the PoP manual that goes over army comps at different stages in the game. In the most simple terms cav dominate small forces, archers dominate medium sized forces, and larger forces are more balanced. With obvious differences depending on what you are fighting.