Great britain and other industrialized nations of the late nineteenth and early twentieth centuries built empires in order to

Obtain resources and markets

Imperialism followed industrialization so countries could obtain resources for industry and have a guaranteed market for their products. 

As countries found need for natural resources, they needed to find cheap way to obtain these resources. The cheaper the resources, the more money the country could make which ultimately gave them more world power. To stay competitive in the market, imperialism allowed for cheap or free resources at the consequence of taking over other areas. 

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.