Where art thou, Muse, that thou forget'st so long

To speak of that which gives thee all thy might?
Spend'st thou thy fury on some worthless song,
Darkening thy power to lend base subjects light?

Return, forgetful Muse, and straight redeem
In gentle numbers time so idly spent;
Sing to the ear that doth thy lays esteem
And gives thy pen both skill and argument.

Rise, resty Muse, my love's sweet face survey,
If Time have any wrinkle graven there;
If any, be a satire to decay,
And make Time's spoils despised every where.

Give my love fame faster than Time wastes life;
So thou prevent'st his scythe and crooked knife.

—“Sonnet 100,”
William Shakespeare

What is the primary purpose of the couplet in this sonnet?

The purpose is to show the poet’s belief in a grim reaper who wields a scythe.
The purpose is to complete the 14 lines required to make the poem a sonnet.
The purpose is to add a twist to the ideas about time described in the three quatrains.
The purpose is to inspire a poem that will help the speaker’s beloved become famous and live forever.