In order to understand the reason for Robert Beverley's Treatise, it is important to review a few historical facts:
The first forced workers in the Southern Colonies were not African in nationality. Well before the use of African Slavery, the southern plantations created contracts with indentured servants. These contracts were not fair and resulted in a forced servitude on the landowner’s property until the ever-increasing debt was paid. Life expectancy on the early plantations was less than six years on average. As the number of indentured servants decreased due to increased (illegal but understandable) emigration to the middle colonies, death, and fewer citizens in England willing to enter into such a contract, African slavery was adopted.
Early African slaves in Virginia were treated like indentured servants. Mostly this was due to the Spanish. They tended to baptize slaves in Africa before putting them on the ships. According to English law, Christians were exempt from slavery. Like the white indentured servants, these Africans were able to pay off their debt and join society as landholders. A good example of this is Anthony Johnson who was an African, was forced to be a slave/indentured servant in Virginia in 1621, earned his freedom from servitude, purchased property in Virginia, and eventually owned his own slaves.
Virginia had no laws concerning slavery until 1640. They started with the trial of an African indentured servant, John Punch, and two other white indentured servants who ran away from their contracts. For breaking his contract, Punch was sentenced to slavery. The two white men only had an extra year added to their contract. The reason: European vs. African heritage. This decision set the dividing line between whites and blacks that would determine Virginia law in the future.
Other trials gradually stripped away indentured servitude from Africans. Virginia changed the status of African indentured servants to slaves in 1661. The Elizabeth Key trial, in 1662, established the precedent of partus sequitur ventrum, which stated that children born to a slave are a slave regardless of the father's nationality. (Key successfully convinced a court that her son was English born because his father was an English citizen. She also became free during the same trial by proving she had been baptized, thus she also was a free woman.) In 1667, Virginia established a law stating that baptism would no longer free a slave by making them a Christian. In 1676, it became illegal for free African colonists to own slaves on their property. In addition, in 1690 it became illegal to marry an African American in Virginia- the punishment: banishment.
In 1705, Virginia established the Virginia Slave Code. It was the first piece of legislation that fully established slaves as property. As property, an owner of a slave could trade them through bartering and use them to pay debts just like money. It also established that a freed slave could be re-enslaved, if the owner needed the money to pay off his or her debts. Under this new law, things became confusing in Virginia regarding the difference between servants and slaves.
This leads to Robert Beverley's treatise. Beverley's document attempts to differentiate between the groups of freed Africans, enslaved Africans, partial African citizens, partial African slaves, and white indentured servants. It is important to note that Beverley did not simply standardize the difference by stating skin color or nationality, but instead, defined the difference between the two by the job description and rights afforded to the individual.
Beverley's document is significant, because it was necessary. Laws and politics like the citizens in Virginia were mixed. The document itself is an attempt to standardize the difference in what constituted a slave and a servant and set regulations for each. It creates a series of qualifiers that differentiated between servants and slaves and established the rights of servants.
Last, Beverley's document was an explanation of what servitude is like to those who lived outside of the colonies:
"Because I have heard how strangely cruel, and severe the Service of this Country [use of slaves and servants in Virginia] is presented in some Parts of England, I can’t forbear affirming that the Work of their Servants and Slaves is no other than what every common Freeman does."
The implementation of the 1705 Slave Codes in Virginia led to many questions in England. (England, itself, still participated in slavery and wouldn't take the first action to abolish it until almost seventy years after this event.) Because of the laws, gradually separating African Slaves from traditional Indentured Servitude, the rights of Indentured Servants were questioned in England, and there was concern over what the 1705 law would mean for white servants who were not part of an indentured servant contract. Beverley's treatise was an attempt to calm those fears by explanation; thus, he focuses most of the document on the legal rights of servants rather than giving a personal opinion on the morality or immorality of slavery.