How does the structure of Sonnet 18 influence its content?

As you'll find in the analysis for this sonnet (see link below), the way Shakespeare would pose a rhetorical question in his sonnets, then only allow for one answer (his own), was important in getting the point of his sonnet across to his reader, and most especially, to the intended recipient of the sonnet.

Shakespeare used the traditional sonnet form for all but three of his sonnets, which is 14 lines, broken into three quatrains (stanzas with four lines) and one concluding couplet (two lines with end rhymes). The rhetorical question is typically posed within the first quatrain, then expounded upon through the other two quatrains, and finally, a conclusion/answer is offered in the couplet.

This form - 14-line sonnet in iambic pentameter - was very conducive to the meaning of the sonnet. It works particularly well with Sonnet 18, as he is positing the fact that his beloved, when compared to the beauty of nature, is far more lovely, more calm, etc. Shakespeare is also making the point that his beloved can be immortal, despite the usual deteriorating effects of aging and nature. He presents this in the quatrains, leading up to his final point in the couplet, which reads:

"So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee."

He is saying at the end that as long as his sonnet exists, and as long as people are still living and can read it, his beloved will be immortal through the lines of his sonnet.