Why is Okonkwo's defeat of Amalinze the Cat such a great achievement?
Okonkwo's defeat of Amalinze the Cat is such a great achievement because it was a display of physical prowess that earned Okonkwo the enduring respect and admiration of his fellow villagers. The defeat also brought fame and honor to the entire village on Okonkwo's behalf. The achievement was also rather impressive because Amalinze the Cat had been undefeated in wrestling for seven years prior.
In relation to the overall plot of the novel, this wrestling victory is so important because it defines Okonkwo and characterizes him as someone fierce and powerful, competitive and masculine, and defensive.
Keep in mind, also, that Okonkwo is particularly anxious to achieve personal fame and honor because he's so ashamed of his father, Unoka, who in life cared more about music and having a good time than being a successful earner--and who, in fact, was deeply in debt to many people. Okonkwo's physical victory in wrestling is one way that Okonkwo rises above the shame cast on him by his disappointing father. (Other ways include amassing wealth, titles, and wives.) Although the narrator of the novel tells us toward the end of Chapter 1 that "among these people a man was judged according to his worth and not according to the worth of his father," Okonkwo still feels that he has to prove himself to the other villagers.