Cities like Timbuktu and djenne were important because

Because Timbuktu was the starting point for trans-Saharan camel caravans which transported goods northwards. Timbuktu was one of the most important cities in the Mali Empire because of its location near the Niger River bend and so it was fed by the trade along both the east and west branches of this great water highway. And Djenné was important because it became a market centre and an important link in the trans-Saharan gold trade. In the 15th and 16th centuries, it was one of the centres for the propagation of Islam.