Slavery is one of the oldest of human practices and virtually every premodern state in history depended on it to some extent. The African slave trade must be seen as part of the larger commercial system of Atlantic trade between Europe, Africa, and European colonies in North and South America and the Caribbean. The system was directed to exploitation of the New World, and thus, the slave trade grew.
In Europe African slaves were preferred over the Indians who were almost exterminated by the harsh climate and treatment. It was not that any particular race should be slaves but more so who could perform the most work under the worst conditions. The purpose of slavery was based on getting rid of work and forcing the hideous labor upon someone else. Africans became the preferred people to enslave because of their physical endurance.
A second reason for the purpose of slavery is simply greed. English and American men saw a way to profit their pockets without having to do any work themselves. Transporting slaves began as a entrepreneur occupation for British seamen, starting with Sir John Hawkins in 1562. It expanded to include England’s greatest trademark, tea. Since tea is the national beverage of English businessmen needed to keep high quantities of their products. These products were largely produced in the tropics. European workers did not have the physical capability to sustain working in such harsh conditions.
Slavery caused over 400 years of suffering and degradation to the Africans, destruction of much indigenous manufacturing in Africa and displacement of many Africans in west and east Africa during the period of the trade in slaves within Africa and around the world. It also led to the division of Africa between the European powers at the Berlin Conference in 1885, ignoring previous historical boundaries, language groups, kingdoms. The spread of racist ideology to justify the enslavement of Africans also set in.
The negative impact slave trade on Africa can be seen on the personal, family, communal, and continental levels. Millions of able-bodied individuals were captured and transported, the death toll and the economic and environmental destruction resulting from wars and slave raids were startlingly high. In the famines that followed military actions, the old and very young were often killed or left to starve. Forced marches of the captives over long distances claimed many lives hence the population of Africa remained stagnant until the end of the nineteenth century.
Besides, the slave trade, led to profound social and political changes. Many communities relocated as far from the slavers’ route as possible. In the process, their technological and economic development was hindered as they devoted their energy to hiding and defending themselves. The relationships between kingdoms, ethnic groups, religious communities, castes, rulers and subjects, peasants and soldiers, the enslaved and the free were transformed.
European and American powers intervened in political process to prevent the rise of African centralized states that would hamper their operations therefore; the slave trade left the continent underdeveloped and vulnerable to colonialism.

From research, Williams’s argument that slavery was not born of racism but a consequence of slavery is factual. The colonial authorities, always faced the possibility of revolt from African slaves and this terrified those in authority hence laws were passed to enforce racial segregation. The laws created racial solidarity among the white colonists. Whites identified themselves as a part of the privileged race exempted from slavery and granted certain civil rights on basis of their color. In these ways slavery was crucial to forming the new racial identities in the American colonies.

Reference

Joseph E. Inikori (1992) The Atlantic Slave Trade: Effects On Economies, Societies And Peoples In Africa, The Americas, And Europe History: African-American Studies. Duke University Press: Durham