The fiction of Thomas Hardy is concerned with the destiny of common people in the grips of an inexplicable fate. What he termed the Immanent Will is an all-inclusive mind or ultimate reality of the universe that induces people to act on impulses or instincts that they are unable to resist. Shaped by a fate that is cosmic in scope, Tess Durbeyfield is "a Pure Woman" who is unable to escape her destiny; victimized, she becomes but a witness to her own fate.
Throughout her life, Tess is a conduit for powerful and dangerous chance forces of Nature. She is forcibly impregnated, she loses her child, her family loses their home, she loses her husband, and is victimized a second time by Alec d'Uberville, who blocks Tess's final opportunity for happiness. In a passive recognition of her fateful position, Tess tells Alec,
"Whip me, crush me....I shall not cry out. Once victim, always victim--that's the law.
"Oh, yes, I have lost him [Angel] now --again because of--you! ...And my sin will kill him and not kill me! Oh, you have torn my life all to pieces--...oh, God--I can't bear this! I cannot!"
Fate, whether in the form of chance and coincidence or nature or otherwise, plays a crucial role in the life of Tess and effects the tragedy of Hardy's novel. In his essay, "Tess of the d'Urbervilles- Repetition as Imminent Design," J. Hillis Wills writes of Hardy's concept of fate,
The various causes proposed have been social, psychological, genetic, material, mythical, metaphysical, or coincidental. Each such interpretation describes the text as a process of totalization from the point of departure of some central principle that makes things happen as they happen.
Thomas Hardy did use his novel Tess of the d'Urbervilles to showcase some obvious forces and philosophies which are in conflict with one another, and of course he did this deliberately. Often an author uses his writing to make a statement about the time in which he is living, and that is the case with Hardy. Many of the conflicts in Tess are reflections of the changes (and thus conflicts) of England during the end of the Victorian age.
One of the primary tensions in this novel exists between the city and the rural areas. During this time in history, England experiences an astounding influx of people from rural areas into the cities because of the beginning of the country's industrialization. Though this was touted as a benefit for all, it clearly was not a benefit to everyone, and that is reflected in this novel.
Notice that the rural inhabitants are the ones who are suffering from the migration to the cities. Hardy takes the side of the rural remnants. For example, when Tess is pure and innocent, her name is Durbeyfield, which means "of the field"; when things change, her name is connected more to the city. Angel gets in trouble in London (the city, of course), and when Tess and Angel take the milk to the train, Hardy's imagery indicates the rift between city and country:
Modern life stretched out its steam feeler to this point three or four times a day, touched the native existences, and quickly withdrew its feeler again, as if what it touched had been uncongenial.
While modern life is disgusted when it has to touch the "uncongenial" rural area, Hardy depicts the industrialized world as some kind of a creeping insect, encroaching where it does not belong.
Notice, too, that the industrialization that finally comes to the farm is a thresher he calls the "red tyrant," and Hardy is disgusted that the women now have to be subservient to a machine.
Another tension in the novel is between religion and morality. Darwin's famous work had just been published, and the country was questioning their faith (including God and religion) in light of the new evolutionary thinking. In this novel, Hardy pits the immorality of his characters against the judgment of the church and its adherents. Notice that all of the characters associated with religious institutions are hypocrites or worse, except perhaps for Angel's father. Notice, too, that while Tess was seen as a fallen woman, the brothels in the cities were doing a thriving business, another commentary on the country/city argument as well as the morality/religion debate.
On a more philosophical level, this novel is also a commentary on the debate between fate and free will. Tragedy after tragedy happens, but do they happen because of who the characters are and their family histories, are do the characters maintain total or any control over their own destinies? Hardy seems a bit conflicted about this one.
Hardy is sympathetic to his women characters, and they are all rather connected to the land; it is the men who are attracted to industrialization, and their lack of connection to the earth seems to be the cause. This is a gender conflict. He is also more sympathetic to individuals than to the collective.
Your question is why he created these and other tensions in the novel, and the answer is that he is trying to say something. He obviously takes a position on these conflicting issues, and we can tell his views in nearly every instance. He speaks his views and expresses his preference through the novel.