Final answer:
The couplet in Shakespeare's "Sonnet 100" aims to immortalize the poet's beloved through verse, defeating the destructive power of time.
Step-by-step explanation:
The primary purpose of the couplet in William Shakespeare's "Sonnet 100" is to provide a resolution to the themes and ideas presented in the previous quatrains, specifically the power of the poet's verse to immortalize the beloved against the ravages of time.
The couplet concludes the sonnet with a powerful hope that the poet's words will give his love fame that outpaces the withering effects of time, thereby thwarting time's destructive power.
Therefore, the poet is not merely completing the structural requirements of the sonnet but is also delivering a profound idea that poetry has the strength to conquer temporality and preserve love.