Published in 1609, the first folio of the sonnets was published during his lifetime, around the time of his retirement from the theatre. They pre-date the publication of his plays (1623), which were published seven years after his death. Ironically, many of the poems are about the immortality of art, and yet Shakespeare seems to have written them without much of a push to get them published. The monarchs Elizabeth I and James I established a haven for art and artists (even non courtiers) to publish works openly, so the spirit of the Renaissance encouraged Shakespeare immensely.
The subjects of the sonnets can be categorized thusly: 1) pure love; 2) tainted love; 3) love betrayed but redeemed by sacrifice. (See www.shakespeares-sonnets.com for clarification).
Many are addressed to men and women. This is important because it reflected an openness to sexuality (even possible homosexuality), which was unprecedented. Also, none of the poems are about God, religion, or the church. As you know, this was a time of great religious upheaval, and to write such secular poetry sets Shakespeare apart from his contemporaries. Not since the Greeks and Romans had there been such secular poetry. It also paves the way for the Romantics, Wordsworth in particular, to wrest poetry from the bourgeoisie and re-define poetry for the common man.
T.S. Eliot has written the definitive essay on the sonnets. Here's an abstract which deals with why the sonnets are important of social change:
In his famous essay on Hamlet, T. S. Eliot referred to Shakespeare's Sonnets as 'full of some stuff that the writer could not drag to light, contemplate, or manipulate into art.' In his ranking of the Sonnets with Hamlet ('Hamlet, like the Sonnets'), he seems to have a critical view on the artistic perfection of the Sonnets, though, probably, not so critical as to label them as 'artistic failure.' What does this rather obscure comment on the Sonnets imply? His comment involves his attitude about the problem of personal expression in poetry, and is related to his early poetic, and his later dramatic, method. The implications of his comment on the Sonnets can be seen on several levels. First, the level of the poetic style in which Shakespeare was gradually reaching for the complex expression of his mature plays but had not yet completely mastered it. Second, the circumstances in which the Sonnets were written, circulated among 'his private friends' and published. That that they are not necessarily in 'the right order' and do not make a 'sequence', that they are different in style and quality, suggests an attachment of the author with the material, a lack of artistic ordering and perfection in which the poet's personal emotion and poetry should be separated. Third, the peculiar quality of the Elizabethan sonnet sequence as an artistic medium, in which 'the man who suffers' and 'the mind which creates' are not completely separated, in which the convention is not perfected as in Dante's Vita Nuova. This point relates to the themes treated in the Sonnets, and also in the Problem Plays, in which the themes are displayed as problems unsolved rather than given artistic resolution. Eliot was deeply concerned with these themes, but he sought for their expression in his own self-conscious way, beginning with the use of personae in his early poetry, and later expressing them through the characters of his dramatic works. His comment on the Sonnets may be considered as a by-product of his quest for the proper medium to express these themes