There are obviously lots of ways to write a book to inspire people to change the injustices of the world around them. One particularly common way is through critical realism, where an author tries to portray a problem or group of problems in society in as authentic and genuine a way as he possibly can. Upton Sinclair's novel The Jungle, which shows the dangers of urban poverty and poor working conditions in early 20th-century Chicago, would be one example of this kind of serious, realistic approach to talking about economic and political issues.
What makes Cry, the Beloved Country so unusual is that, like The Jungle, it is all about social problems. But it does not take the same realist approach to style. Instead, Cry, the Beloved Country is almost like a folktale, since it can be ambiguous in places. For example, here's a passage from Kumalo's final visit to Absalom's prison before he goes back to Ndotsheni:
They passed again through the great gate in the grim high wall, Father Vincent and Kumalo, Gertrude and the girl and Msimangu. The boy was brought to them, and for a moment some great hope showed in his eyes, and he stood there trembling and shaking. But Kumalo said to him gently, we are come for the marriage, and the hope died out. (2.29.1)
Check out that first sentence: "great gate in the grim high wall." Paton is using alliteration, which means that he is repeating the first sound in a sequence of several words. This pattern makes his sentences sound almost sing-songy, like a story that's being sung rather than spoken plainly.
Paton's style comes across as highly poetic both because he uses literary tools like alliteration and because he often avoids giving us specific details about where and when the action of the novel is taking place. Where is that "grim high wall"? Before Paton makes it clear from context, we can only guess that it is Absalom's prison wall. But we can't be a hundred percent certain about that from the first sentence; Paton deliberately makes this ambiguous and confusing.
Speaking of deliberate confusion, Paton also uses "the boy," even though we all know he means "Absalom." "The boy" is a strange way to refer to a character whom we already know, after all. But calling Absalom by name would make it clearer that he belongs to a particular time and place. By leaving his name out of it, Paton makes this scene feel uncomfortably closer to the readers. We could fit any boy we know into "the boy," where "Absalom Kumalo" can only be one person. Paton uses deliberately mysterious, generic words and scene settings to make the events of Cry, the Beloved Country more unsettling to the reader. And yet, at the same time, more universal.