Why would a poet use various rhyme schemes?

I assume you are asking why John Milton uses the rhyme schemes that he does in this poem. Milton uses two different poetic forms in this poem. In the first four verses he uses the poetic form called "rhyme royal," which was first used in the English language by Geoffrey Chaucer in his narrative poem Troilus and Criseyde. Rhyme royal consists of seven lines with ten syllables each, though some poets extend the last line to twelve syllables, as Milton does in this poem. Each line, including the last one, is in iambic pentameter, where every even-numbered syllable is accented, while every odd one is not ("This is the month, and this the happy morn"). The rhyme scheme is as follows: ababbcc, where every letter stands for a different rhyming sound. Milton seems to have chosen this form based on its elegance and grace. This form emphasizes the peaceful joy that comes with the particular day on which Milton writes.
The other twenty-seven verses of this poem consist of a different rhyme scheme: aabccbdd. Each line with the a-rhyme contains six syllables and each line with the b-rhyme contains ten, while the last two lines contain eight and twelve syllables respectively. It is unclear as to why the poet chose two different rhyme schemes for his poem, but the second form adds more of a song-like tone, which fits with the overall theme of rejoicing on such an occasion.