Letter to Sir Walter Raleigh

Reading this letter was a challenging task. Most of the words are known nowadays, but I felt there was some information missing. I felt I did not get the whole idea from the letter, neither the purpose.
I believe the author wanted to use a historical character to develop his story of a noble and virtuous gentleman. At some point, this story is called an allegory because it tried to make a figurative representation of something else; something abstract that it is better understood by using what we know as allegory. For instance, using some well-know character to interpret the main values or virtues of this invented gentleman who is the main character of the story or the usage of king Arthur who we all know from real and fictional stories.
Moreover, Sencer used many characters such as King Arthur and some knights to show some characteristics that he considered important: virtuosity, bravery, honorability and nobility. The author used 12 knights to represent 12 different vales and their opposites to improve his story.

I think this choice was be made base on the common knowledge the society had about this stories, for example, King Arthur's.

I am not sure why he wrote this letter, however, I am going to make a guess. Mr. Spencer would had written to Sir Walter Raleigh because he wanted to show respect to him and at the same time, validate his work in front of his eyes.