In lines 41-60 of Book 9 of the Odyssey, the author discusses the sweet liquor that Euanthes's son gives to Odysseus. The main purpose of this discussion is to show Odysseus's cleverness and ability to outsmart his hosts.
Odysseus had just escaped from the cave of the Cyclops and was seeking hospitality from the people of the island. Euanthes's son offers him a sweet liquor that he says will take away his sorrows, but Odysseus suspects that it may be a trap to make him forget his troubles and reveal his true identity. Therefore, he pretends to drink the liquor but actually pours it out when Euanthes's son is not looking. Later, when the other islanders arrive and ask him why he is not affected by the liquor, he uses his wits to make up a convincing story about his exceptional tolerance to alcohol, further impressing his hosts with his cunning and resourcefulness.
Overall, the main purpose of discussing the sweet liquor is to highlight Odysseus's intelligence and quick thinking, which are key traits that enable him to survive and overcome the many challenges he faces on his journey back home.