Why are couplets used?

Answer 1

Couplets can be used in multiple forms of poetry. For example, a Shakespearean Sonnet makes use of it. A sonnet has three quatrains followed by a couplet. The rhyming scheme is ABAB, CDCD, EFEF, GG. Here's an example of one of Shakespeare's sonnets. Notice the final two lines as a couplet:

My mistress' eyes are nothing like the sun;

Coral is far more red than her lips' red;

If snow be white, why then her breasts are dun;

If hairs be wires, black wires grow on her head.

I have seen roses damasked, red and white,

But no such roses see I in her cheeks;

And in some perfumes is there more delight

Than in the breath that from my mistress reeks.

I love to hear her speak, yet well I know

That music hath a far more pleasing sound;

I grant I never saw a goddess go;

My mistress when she walks treads on the ground.

And yet, by heaven, I think my love as rare

As any she belied with false compare.

Couplets are also used on their own in order to make a quick little poem with a simple but clever structure.