What is the main purpose of Sonnet 18 by William Shakespeare?

The main purpose of Shakespeare's Sonnet 18 is embodied in
the end couplet:

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

The sonneteer's purpose
is to make his love's beauty and, by implication, his love for her, eternal. In doing
so, he takes a rather circuitous pathway by beginning with a comparison that does not
describe her.

In the first two quatrains, he is essentially saying,
"No, I cannot compare you to a summer's day, and here's why." The last two lines of the
first quatrain,

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Rough winds do shake the darling buds of
May,
And summer's lease hath all too short a
date:

along with the whole
second quatrain (four lines of poetry with alternating rhyme) explain why she can
not be compared to a summer's day. For
instance, "rough winds" do not shake her beauty and her
beauty is not sometimes dimmed as even the mighty sun's
glory is sometimes dimmed:

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[that] too hot the eye of heaven
shines,
And often is his gold ...
dimm'd;

The third quatrain,
beginning with the contradictory conjunction "but," begins
the explanation that leads up to the main purpose expressed in the ending couplet (two
rhyming lines of poetry). In this final quatrain, the sonneteer says "But ..." your
beauty shall be an "eternal summer" and shall not fade nor be dimmed so that even Death
shall not rob your beauty.

Then comes the implied "because" of the
couplet that explains that as long as Sonnet 18 can be read, then she will have beauty
and she will have life eternal.