Why was straw added to mud bricks in early river-valley cities?

In early river-valley cities, straw was added to mud bricks to serve as a reinforcing agent. As the bricks dried in the sun, the clay mixture would inevitably shrink, leading to the formation of cracks. The straw acted as a binder that held the brick together during this curing process, preventing structural failure and ensuring the bricks maintained their integrity as they hardened into a uniform mass.12