Life for black people in South Africa is cheerless. The people are oppressed and they lack prospects for a good ife. As far as work goes, they can have only the menial jobs. By law, black people must carry passbooks without which they cannot get work, or move about without getting into trouble with the authorities.
This is the society in which Sizwe finds himself when he arrives in Port Elizabeth without valid papers. Therefore, his stay there is considered illegal, even though he is a native of the land. His discovery in Zola's place leads to his finding refuge in Buntu's place.
While returning with Buntu from an evening out, he encountered a dead man on a rubbish dump. This leads to his so-called "death". Buntu comes up with the idea that Sizwe swap identities with the dead man. He even explains that the dead man would have approved Sizwe's adoption of his identity. With the replacing of the dead man's picture in his passbook with Sizwe's, Sizwe Bansi becomes Robert Zwelinzima. The best thing that happens in the life of Sizwe is this "death".
The immediate significance of this "death" is that it enables Sizwe, alias Robert, to avoid imminent repatriation to King Williams's town. Thus he can now live in Port Elizabeth by right and by preference. His "death" also entitles him to work and to have some marginal dignity in the society. It is ironical that it requires Sizwe's "death" in the manner it occurs for him to survive in his own country.