A couplet is a pair of successive lines of metre in a poem or a song. These lines usually rhyme and share the same metre. In this case, both "To my Dear Loving Husband" and "To the King's Most Excellent Majesty" employ a couplet. This couplet is located at the end of the poems. There are two main reasons why the couplets are used in this way. First, the couplet is meant to provide a sort of "conclusion" or "answer" to the questions posed in the rest of the poem. The second reason is that the couplet is meant to add to the melodic rhythm of the poem by concluding with a rhyme. The effect of this is making the poem more memorable to the reader.
Traditional poetry usually has regular rhythms that set a pattern, making it easier to remember for recitation. These rhythms are especially enjoyable, because they reflect the natural movement of the human body. That's why poems with regular rhythms are often set to music for marching and dancing, amplifying the effect of the beat.
So basically, the rhyming and rhythms in these two poems makes the poem flow better and sound better. It also makes it easier to memorize.
