Things Fall Apart - W.B. Yeats poem

Here is the full version of "The Second Coming" by W.B. Yeats - four lines from it are included in your book before the start of the story. Why was this used by Achebe? What questions does this raise? What layers of meaning does it add?

"The Second Coming"

Turning and turning in the widening gyre1
The falcon cannot hear the falconer;
Things fall apart; the center cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.

Surely some revelation is at hand;
Surely the Second Coming 2 is at hand;
The Second Coming! Hardly are those words out
When a vast image out of Spiritus Mundi3
Troubles my sight: somewhere in sands of the desert
A shape with lion body and the head of a man,
A gaze blank and pitiless as the sun,
Is moving its slow thighs, while all about it
Reel shadows of the indignant desert birds.
The darkness drops again; but now I know
That twenty centuries 4 of stony sleep
Were vexed to nightmare by a rocking cradle,
And what rough beast, its hour come round at last
Slouches towards Bethlehem to be born?

- William Butler Yeats (1921)

Notes:

1. Spiral, making the figure of a cone.
2. Second Coming refers to the promised return of Christ on Doomsday, the end of the world; but in Revelation 13 Doomsday is also marked by the appearance of a monstrous beast.
3. Spirit of the World.
4. 2,000 years; the creature has been held back since the birth of Christ. Yeats imagines that the great heritage of Western European civilization is collapsing, and that the world will be swept by a tide of savagery from the "uncivilized" portions of the globe.

Yeats was attracted to the spiritual and occult world and fashioned for himself an elaborate mythology to explain human experience. "The Second Coming," written after the catastrophe of World War I and with communism and fascism rising, is a compelling glimpse of an inhuman world about to be born. Yeats believed that history in part moved in two thousand-year cycles. The Christian era, which followed that of the ancient world, was about to give way to an ominous period represented by the rough, pitiless beast in the poem.