In past battles, once the fighting started, it was very difficult for commanders to take control of the entire battlefield unless they found a particularly high and open vantage point from which they might be able to overlook the whole field.
But most of the time, the rules were set before the battle began, and then everyone acted according to the plan during the fight, making it hard to adapt spontaneously.
Even if you wanted to change orders, the time it took for commands to issue from the commander's mouth to reach the ears of the frontline soldiers was very long.
So once the battle began, unless it was the most elite troops, it was also hard to make changes on the spot. If you just acted randomly, you would first disrupt your own formation, and the soldiers wouldn't understand all that; if you kept changing orders, they would become unable to react.
