First of all, salt is a necessity for the people, but the people can't produce it themselves, so they have to buy it. Controlling salt means indirectly controlling people. For example, when exploitation was serious in ancient society, there were many "refugees". Refugees are those who leave their household registration in order to evade taxes. However, both refugees and good citizens always want to eat salt. If the state monopolizes the salt industry, these refugees will eventually return to the government to buy salt. As long as salt is taxed, the purpose of controlling people and ensuring tax revenue can be achieved.

Secondly, everyone wants to eat salt. The market is very broad and the business opportunities are huge. But the threshold of salt industry is very high, and not everyone can sell it. Only powerful families can engage in salt sales, so these people have a good chance to sit on the ground and raise prices, and often get dozens of times of profiteering. In the early Han dynasty, the government allowed the people to sell salt, and many powerful families were born. They are as rich as the enemy and in a dominant position. Later, the government took over the salt industry. First of all, it can make huge profits. Secondly, it can crack down on powerful families, narrow the gap between the rich and the poor, ease the class contradiction between landlords and peasants, and be conducive to the stability of political power. Since then, the rulers of all dynasties have become smart and never let go of the salt industry easily.