The existence of set piece battles absolutely did happen. In general an army could raid an alternate army with cavalry, but it couldn't force another army into a battle it didn't want. Small scale ambushes were fairly common but full on army to army ambushes like Lake Trasimene were relatively rare.

More common would be an army would assemble and "offer battle" and wait for the other army to also assemble. Attempting to rush an assembling army, would simply cause that army to withdraw. And infantry chasing infantry would be a great way to be led into an ambush. It's hard to catch an unwilling army on open ground, before they can find a strong defensive position.

So as this assembling and preparation was going on there was time for a leader to give a quick speech or other demonstration. The literature of ancient and medieval battle reports are so filled with pre-battle speeches that they had to actually happen. Maybe the official versions were prettied up a bit but they almost certainly did happen. And the practice carried on into modern times. General Eisenhower for example provided a lovely little speech on D Day in his Order of the Day. Of course he had radio and could reasonably expect everyone to hear him. There's good reason to expect that most soldiers in ancient or medieval times only had part of the gist of the message repeated to them. Caesar's speeches were likely only heard by the closest few hundred men.