How did trade encourage the growth of Djenne - Djeno?

It is suspected that Djenné-Djenno grew to such a vast size as a result of regional and local trade. Djenné-Djenno was part of a pre-Arab trans-Saharan trade network. It has been hypothesized that the city grew as a trade center due to its location on the southern portion of the agriculturally productive region of the delta.