Viewed from a historical context, Wilde's Victorian society, from the 1860s to 1900, intended for everything, printed or published, to teach a "moral" to audiences. However, the Aesthetic movement that pre-Raphaelites such as Wilde, Pater, and many others followed, was meant to be amoral; not moral nor immoral. This means that aesthetes believed then (and still do now) that the job of the artist is to manifest and not to teach; to create, and not to debunk; to inspire beauty, instead of analysis.
This being said, Oscar Wilde wrote the preface to The Picture of Dorian Gray after the first publication of the novel in Lippincott's magazine (1890) received a notoriously bad reception by the Scotts Gazette, and by the St. James's Gazette, among some of the most influential publications of the time. The novel was dubbed everything, from "trashy", to "immoral", to "dangerous", and even "criminal".
Wilde wrote his preface using epigrams and paradoxes. These are plays on words that illustrated a mentality which was quite new for Victorian London. The paradoxes were meant to sound contradictory; yet, Wilde's genius was such that he made the contradictions reinstate his views on Arts and aesthetics even further. An example is found in these phrases and philosophies in the preface:
All art is at once surface and symbol.
Those who go beneath the surface do so at their peril.
Those who read the symbol do so at their peril.
One would think that Wilde is contradicting his views because he is giving Art a superficial value; as if Art only has skin-deep qualities.
However, what Wilde is actually saying is that Art is not meant to teach, condone, nor condemn anything: Art is a conduit of beauty, not a moralizing medium. When people try to understand or view Art within a didactic context, Art loses its beauty, and its purpose, altogether.
This is because only Art gives us the freedom to manifest everything we create, love, hate, want, or wish for. If we corrupt these freedoms by adding the connotations that come with morality, we are literally ruining the essence of Art.
These are also the reasons why Wilde re-emphasizes in the end:
The only excuse for making a useless thing is that one admires it intensely.
All art is quite useless
Sounds contradictory, but it is not. By "useless" Wilde reinstates his view on Art as the only thing in life whose purpose is to produce admiration and awe, instead of guilt or redemption. We should not "use" Art; instead, we should admire it, and enjoy it. Therefore, Wilde does not contradict himself. Instead, he adheres with strength to his ideals.
Conclusively, as a life-long enemy of critics, Wilde's prologue uses irony, sarcasm, axioms, paradoxes, and epigrams in his preface in order to confuse his critics, demonstrating that they cannot grasp the intellectual nature of a novel like Dorian Gray, and much less of the Aesthetic movement.
Just when we think Wilde is about to fall flat in his words, he resurfaces with both skill and intelligence. In his time and place, his style of writing and speaking was, perhaps, the only way that he could try and tame the bands of "philistines" (his favorite name for critics and ignorants) that attacked him until his death.