Final answer:
King Afonso I's letter was written to persuade the King of Portugal to provide assistance to his kingdom, addressing the negative impacts of the slave trade and a public health crisis. The purpose of historical letters is often to inform or persuade their intended audience.
Step-by-step explanation:
The primary purpose for King Afonso I of the Kongo writing his letter to João, the King of Portugal, was to seek aid and remedy for the problems that were afflicting his kingdom. He was addressing issues that ranged from the social disruption caused by the slave trade to the public health crisis due to a lack of medical professionals and facilities. Furthermore, he sought the benefit of his kingdom and the welfare of his subjects by requesting skilled personnel to address these urgent needs. The tone of the letter indicates that its purpose was to persuade the Portuguese monarch to take action by providing the requested support.
In the case of other historical documents, such as Luther's Address and Charlemagne's writings, the purpose can differ significantly. Luther's Address might have been meant to persuade the German nobility, while Charlemagne aimed for moral and cultural reform of his subjects and the realm. The general utility of letters and documents like these lies in their capacity to inform, persuade, and guide their readers, be they laypersons, clergy, or rulers themselves.