Why does Chinua Achebe tell us the story of how Chike's parents were married in the middle of "Chike's School Days"?

In the middle of the story, Chinua Achebe relates the circumstances surrounding the marriage of Chike's parents in order to provide us some sort of context for Chike's seemingly strange habits.

Accordingly, Chike and his family live in a traditional Nigerian village where children are regarded 'as the common responsibility of all.' One day, thinking to do him a kindness, a neighbor lady offers Chike, then four years old, a piece of yam. To her consternation, Chike resolutely rejects her gift, telling her that his family does not partake of 'heathen food.' Despite her anger, the woman marvels at the audacity of an Osu in rejecting her gift. At the time, the Osu were considered Nigeria's outcasts. They were forbidden to own land and to participate in common rituals. Upon death, Osu had to be buried on separate land, called the 'Bad Bush.' Anyone who married an Osu was considered an outcast as well (Chike's father, Amos, married an Osu woman despite not being an Osu himself); the children of such a union were also pronounced 'untouchable' and shunned by the general populace.

In the story, the neighbor thinks that the white man's presence in Nigeria has precipitated a state of affairs antithetical to traditional sentiment. To her, the fact that an Osu like Chike would be so bold as to reject a gift from a free-born Nigerian means that the white man has usurped the natural order of things in Nigerian society. She in incensed that an Osu would dare to put on such airs, considering that he is a lowly outcast.

So, Chinua Achebe tells us the story of Amos' marriage to explain Chike's seemingly strange, uncharacteristic behavior and attitudes (at least attitudes strange for an Osu child). He further states that the importance of knowing how Chike's father becomes an Osu cannot be underestimated, as such an occurrence is so rare. Achebe shows us that, just as Amos rejects traditional Nigerian concepts of class, so does his young son. Neither views himself as an outcast. Chike's anglicized view of both religion and education further highlights the fragmentation of Nigerian society and the conflict between the traditional and the new ways.