If you you have another stack of reinforcements, they enter the battle with you as an entire separate army instead of just a trickle when your own units start dying. I don't know if FotS has the same controls, but in Kingdoms you could give your reinforcement army basic commands like wait here, attack here, do what you want, etc.
So you can have 40 v 40 battles instead of just 20 v 20.
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