What does Odysseus gain from fighting Irus in Book 18 of Homer's Odyssey?
In the eighteenth book of Homer's Odyssey, Odysseus returns to his own home disguised as a beggar. Once inside the house, he meets another beggar named Irus. Because Irus begged regularly at the house, he does not want any other beggars to move in on what he considers his territory. Accordingly, Irus the beggar tries to drive Odysseus the beggar away from the palace.
When a quarrel breaks out between Irus and Odysseus, the suitors are amused and want to see the two men fight. The suitors also decide that the winner of the fight can have his choice of the food that they are preparing for their dinner "And more, he shall always dine with us. And we’ll not let anyone else mingle among us and beg" (A.S. Kline translation). Since Odysseus wins the battle of the beggars, this, supposedly, will be what he gains.
Additionally, as winnizhang suggests, Odysseus also makes some inroads with the suitors and works his way into their confidence to some extent.
Odysseus also may gain an additional dose of pity from the gods as he has to suffer abuse in his own house from a beggar. Odysseus' battle with Irus foreshadows the much grander battle he will have with the suitors in Odyssey 22.