Why was trade so important to the development of ghana, mali, & songhai?

Ghana, Mali, and Songhai were rich with gold and built much of their wealth from the exportation of their gold resources. In the 4th-5th Century, this trade gained traction where traders sold to Mediterranean traders. As demand grew for gold across the world, creating a vibrant trade. To these ancient countries, gold created great national wealth, allowing them to expand their territories, and gain much-needed goods from other countries such as salt, and other commodities.