The purpose is to inspire a poem that will help the speaker's beloved become famous and live forever.
Explanation:
Primary purpose a story means the basic reason of the story. Also, it can be referred to as the essence of a story or idea.
From Shakespeare's Sonnet 18, as long as men can breathe or eyes can see; and as long as Sonnet 18 can be read, then she will have beauty and she will have life eternal. This implies that she would continue to be famous and also live forever.