Flying units may move great distances and may cross enemy lands without restrictions. Seas cannot be crossed by flying and still requires a full move to enter or exit. Fliers are also able to launch quick attacks on archers and commanders in battles.
Note: When using flying units in combat, consider having them hold and attack rear rather than simply attack rear, lest nearby units that would otherwise be too far and going for your frontline switch target to your fliers.
Flying units set on a line formation and scripted to attack or fire will walk instead of flying at the start of combat. Hold and fire with a line also keeps them on the ground, but hold and attack wont. Units can always decide to switch to flying later in the fight, after the initial engagement.