“Epithalamion” is a type of lyric poem known as a Spenserian sonnet. 

It was written by Spenser, so that makes sense.   In a Spenserian sonnet, “the rhyme scheme is abab, bcbc, cdcd, ee” (Wikipedia).

Here is an example of the first four lines’ rhyme scheme.

Ye learned sisters which have oftentimes(a)

Beene to me ayding, others to adorne: (b)

Whom ye thought worthy of your gracefull rymes, (a)

That even the greatest did not greatly scorne (b)

The rhyme scheme contributes to the musical quality of the sonnet.  Like other sonnets, “Epithalamion” is a love poem: a wedding song.  A wedding song allows us to rejoice in the happiness of the wedding and celebrate love.  The sonnet is the perfect vehicle for this, because sonnets are traditionally love poems.  A man would sing “Epithalamion” to his love, or write a sonnet of his own.