Answer:
The purpose of the figurative language in the excerpt is to use analogies to represent an idea of masculinity.
Okonkwo is said to be like a "roaring flame", and a roaring flame is supposed to be powerful, a synonym for masculinity.
But he is described as having a son that was "effeminante", and the figurative language used to describe the son is that of "cold, impotent ash" that was left by the "roaring flame" after it stopped burning.