The armor and weapons of the elite troops weigh dozens of pounds. If you let the elite March in armor, they will be tired as dogs when they get to the battlefield. Even the elite can eat well. If they bring their own equipment and food, they will definitely not be able to walk. Elite cavalry needs two horses, or even three horses. If they are allowed to take care of their horses all day, they will become grooms, so they don't have to sleep.
The elite want to attack the city, but ladders and all kinds of equipment need to be made by the army. The elite should hold the barracks. Breaking the city can make the elite rob for three days, but the elite always needs someone to carry it and transport it to the rear. The treasures they robbed need to be rewarded to the elite, and the rest of the good things need to be cleaned up by the old, the weak and the sick. No credit and no hard work. You must deal with it!
Those "wounded soldiers, the old, the weak and the sick" thought that when they were young, they also followed the general to the battlefield and had a narrow escape. Can you kick the generals out after they are injured and disabled? What would the other soldiers think? What would they think? How to mix in the army in the future? With worries, I dare not fight hard. Therefore, in an extremely critical situation, the generals will put the wounded and disabled soldiers into the second-line auxiliary soldiers and give them food, so as to stabilize the morale of the army.
An example in China's history: When Cao Cao was fighting, he often sent large troops to crush the enemy's walls, and then he would forcibly harvest the other side's wheat and take all the materials away. If these wheat cutters are the main force and kill people from the city, you will be cold. So the situation was that the main force put on armor and waited for the enemy to come to the door with weapons, while the people and auxiliary forces went up and cut off the enemy's wheat, all kinds of crops and all the materials that could be taken away. If the city people dare to come out, the main force will come up. If they dare not go out of the city, they can only watch the wheat being cut and the materials being taken away. Keep an eye on this. These auxiliary wheat-cutting troops are all "old, weak and disabled soldiers" and do not need too strong combat effectiveness. Now they are regarded as logistics support personnel, and the troops in front rely on them to keep fighting.