"Sonnet 80" is one of Shakespeare's sonnets in which he alludes to a rival poet:
Knowing a better spirit doth use your name,
And in the praise thereof spends all his might,
To make me tongue-tied, speaking of your fame!
"Sonnet 80" continues Shakespeare's train of thought from "Sonnet 79," and his rivalry with the mystery other poet continues on until "Sonnet 86;" Shakespeare uses the form of the sonnet, because his mystery rival poet clearly also uses the sonnet. Shakespeare wishes to best him in both form and content. Much debate has surrounded the identity of Shakespeare's rival, for he was never outright named in any of Shakespeare's poems or writing. Some critics point to Marlowe or Spenser.
So to return to your original question, the motive behind Shakespeare's style and form in "Sonnet 80" stems from his desire to outdo his contemporaries. They wrote in sonnet form, so he wrote in sonnet form. His style in "Sonnet 80" is slyly humble, likening his poetry to a "saucy bark far inferior to his" (line 7). Shakespeare's nautical humility in this poem is a front, because the speaker's sense of worthlessness is really an appeal for sympathy and ultimately, admiration for his fine writing style.
Rolfe, W. J. Ed. Who was the Rival Poet. From Shakespeare's Sonnets. New York: American Book Company, 1905. Shakespeare Online. 14 Jun. 2012. web.
The Shakespearean sonnet is fourteen lines of rhymed iambic pentameter. They almost all follow the rhyme scheme,
In this particular sonnet, the poet (Shakespeare) is complaining about a rival poet who seems to be able to please the subject of his poem more effectively than he. Your question has to do with style. I'm not sure there is a "reason" other than it just is the style Shakespeare chose for his sonnets. He uses blank verse throughout the plays, which is unrhymed iambic pentameter. It is very rhythmic, like a heartbeat. It was not uncommon for poets to "compete" with each other by writing about similar subjects to see who could write the most pleasing poem. The sonnet was preferred, because it was highly structured and fairly easy to construct.