It's nothing really important or needed, but brings some more realism I think.
We all love carpet sieging. Leaving those adorable 2,000 troops stack (adjusted to the fort level) and watching the strapes to appear, but how should the sieged one feel like? They stand on walls and see what? An equal amount of troops! Why should they let themselves be starved to death? They can try to go out and fight them in a regular battle! I don't know if feature like this would be used more than few times, but if you're in a fight with less advanced enemy you can try to fight them, right?
We all love carpet sieging. Leaving those adorable 2,000 troops stack (adjusted to the fort level) and watching the strapes to appear, but how should the sieged one feel like? They stand on walls and see what? An equal amount of troops! Why should they let themselves be starved to death? They can try to go out and fight them in a regular battle! I don't know if feature like this would be used more than few times, but if you're in a fight with less advanced enemy you can try to fight them, right?