Why did Ghana rules tax Traders passing through the kingdom

In the 400s, a group of primarily nomadic people named the Berbers formed a kingdom just south of the Sahara desert; they called their kingdom Ghana. They began to use camels to carry goods across the Sahara Desert. From the north came salt; from the west came the rich resources of gold, ivory, and other goods. Ghana was located in the middle of the salt and gold trade routes.