What favor does Odysseus do for his men?
This question may refer to Odyssey 12, where Odysseus lets his men know about the various advice and prophesies of which Circe told him. Regarding Circe's words, Odysseus makes explicit mention about the dangers they would face when they passed the island of the Sirens. If Odysseus' men were to hear the Sirens' song, then it would result in their deaths. Accordingly, Circe advises Odysseus to have his men plug their ears with beeswax.
I, in the meantime, sliced a large cake of beeswax with my sword-edge, and kneaded the slivers in my strong hands until the pressure and the rays of Lord Helios Hyperion heated it. Then I plugged the ears of each of my friends... (A.S. Kline translation)
Thus, with their ears plugged, Odysseus' crew is unable to hear the Sirens' song and they are saved from death. This could be the favor to which the question refers. Of course, Odysseus does not tell his crew everything about the journey ahead; he neglects to mention Circe's advice to steer closer to the Scylla than the Charybdis. So, while Odysseus does not lose his entire ship to the Charybdis, the six men he does lose to the Scylla had no idea of the horrific end they would meet.