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.