Things Fall Apart

Things Fall Apart culminates in a tragedy for Okonkwo but then ends by unexpectedly detailing the plans of the District Commissioner, who wants to write about Okonkwo's story, yet "Perhaps not a whole chapter but a reasonable paragraph, at any rate. There was so much else to include, and one must be firm in cutting out the details" (Achebe 209). What is Achebe's purpose for switching the narration from Okonkwo to the District Commissioner, and how is this related to the tragic processes of colonialism in general?