And, by the way, the human faction is a "powerhouse" faction, i.e., they fight with an army more than with magic. Accordingly, leveling up light magic, which strengthens the army, is essential. So the army is an important aspect at any level of hero development.
Heh, this is in the fourth Heroes game; in the last mission, you could walk around with your tank without an army at all – we were still stronger than any army.