This seems like a pretty thoughtful question to me. In Book 12 of Homer's Odyssey, Odysseus and his crew have an encounter with a horrific monster called the Scylla. Circe describes the monster as having twelve legs and six heads.
When Odysseus and his crew sail past the Scylla, Odysseus relates how the monster snatched six of his men off the deck of the ship. Homer compares Scylla's actions to that of a fisherman:
as a fisherman on a jutting crag casts his bait to lure small fish, lowers an ox-horn on a long pole into the sea, and catching a fish flings it ashore (A.S. Kline translation).
Homer's simile here seems to focus on the vast size of the Scylla in comparison to the relatively small size of Odysseus' men. The men are like "small fish" that can be scooped up by "an ox-horn". Thus, the monster is huge in comparison to the men.
Homer's simile also emphasizes the heirarchy of monster to human. The monster becomes the fisherman, while Odysseus' men become the fish.
As is usual with Homer in both the Iliad and the Odyssey, his similes (and his metaphorical language in general) often make the unreal or the unfamiliar real--that is, no one in his audience has ever seen, or will see, a monster like Scylla, but many are familiar with the technique of fishing with an ox-horn sheathe that keeps the fish from biting through the line, and everyone can picture the fish as they struggle once they are caught. Homer, or the poet we refer to as Homer (we're not sure even if there was a Homer), ensures that the image of Scylla destroying Odysseus' men is made concrete by comparing it to a real-life experience that would resonate with many of the poem's listeners.
Perhaps the most interesting aspect of this episode in which the simile plays such a part is that Odysseus knows that his men are likely to be killed by Scylla, but because he wants to get back to Ithaca, he purposely keeps his crew ignorant of their fate:
I said nothing of Scylla, that inescapable horror,/since I was afraid that my comrades, appalled would stop/rowing and dive below the deck and huddle there weeping. (The Odyssey, trans. Stephen Mitchell, ll. 209-211)
At this point in the voyage home, Odysseus has very few of his original crew left, and his decision to jeopardize his men's lives seems harsh, but it is consistent with the Odysseus who has overcome such odds during this voyage, as well as during his ten years of leading his men through the Trojan War.
Homer's simile--in which the agonies of Odysseus men are drawn out in such detail and at length--is meant to create pathos (feelings of sorrow or pity) for both the men being killed and the man who consciously makes the decision to send them to their death. The key word in the quote above is comrades--these men comprise not just a crew but a group who has shared everything for almost twenty years.
In the end, however, the Scylla episode, simile and all, points up the fact that Odysseus is a leader who must live by his decisions, good or bad, which is why he concludes with
That was the most sickening thing I ever saw on my travels. (ll. 240-241)