for what reason did colonial powers mandate export of raw materials in colonies? what labor structure did this establish

Colonial powers designed a very profitable trade model. While they were able to extract cheap raw materials from the territories they controlled, they exported those resources to the metropoli where there was enough technology to elaborate final consumption goods. Those goods had a much larger value than the sum of the value of its raw components, due to value-adding mechanisms, and were subsequently exported again to the colonies generating a large profit for the colonial power.

One of the main reasons why it was so cheap to obtain raw materials in the colonies was because the exploitation was based on slave labor.