Unit Replenishment
Since veterans stay with your avatar from battle to battle, any casualties they suffer will persist. If this goes on, they will become too weakened to fight effectively. However, you can bring them back to full fighting strength by resting them for one battle. Leaving veterans out of your army in this way will mean that they will have been replenished in time for the next battle. For this reason, it is wise to retain several veteran units of each type to allow rotation of forces and keep them at full fighting effectiveness.