The final Battle may indeed be an issue of will, with only Rand's mental state mattering.

But it doesn't begin and end there. The Dark One's power, his ability to affect the world, obviously increases with greater chaos in the world. He is the true Lord of Chaos, after all.

Without war, without huge armies of Shadowspawn majorly threatening humanity, Rand will find it much easier to defeat the DO.

Thus, while these physical battles, leading to the physical destruction of humanity, are not the main aim of the DO, they are critical to his success.