why did achebe add a chapter after okonkwo's death?

That last section ties everything together and allows the reader to understand how and why the story is being told.

"Okonkwo's death may make a lively paragraph in the book he plans to write about the British victory over the savages of Africa. He has already chosen a title: The Pacification of the Primitive Tribes of the Lower Niger."

In this chapter, the author shows us that damage has been done by earlier European depictions of Africans (this is what they talk about in the conclusion; the novel does not follow this perception).