Why did shi Huangdi tear down the walls of Chinese kingdoms

He was the first emperor of the whole China, before the Qin dinasty China was divided in many kingdoms that although sharing the same religious and cultural identity, known as the hundred schools of thought, weren't politically unified. The period between 481 BC and 221 BC is known in chinese history as the warring states period, this was a time of intense warfare between the major states that divided China and saw the Qin kingdom under the rule of Shi Huang victorious, and the Qin became the first dinasty to rule all the Chinese lands.