Symbolism, Imagery, Allegory
Back in the day, sea monsters were huge opponents of the god(s). They had different names and took on different forms (Rahab, the sea dragon, and the Leviathan), but they all played the same role. It was like passing the bar, but for ancient gods—you have no street cred unless you take on this dragon.
In the Book of Job, God shows up late in the game to say his piece about divine power. And how does he do it? Well, he describes his power in terms of defeating the sea dragon. In fact, Job is one of the only places in the Bible where we get a long, healthy description of what this thing looked like, and, by all accounts, it was epic.
Picture the scariest, biggest sea monster you can. Oh, and it breathes fire. Not enough for you? How about this? Its "sneezes flash forth light/ and its eyes are like the eyelids of dawn./ From its mouth go flaming torches;/ sparks of fire leap out" (41:18-19). But wait, there's more: "terror dances before it" and "its heart is as hard as stone" (41:22, 24). Hmmm…what could take on this kind of sea beast? A storm god, perhaps? And that's just what God is.
But why does God spend his time talking about this thing? Maybe because this sea monster embodies untamed, natural chaos; it's malevolent, evil, and totally destructive. God is basically telling Job, "Look, man. I am the only thing standing between you and this vast, powerful creature that wants only your destruction. Choose me. Seriously."
[Other places in the text where Nessy pokes his head out of the water are 3:8, 7:12, 9:8, 9:13, 26:12-13, and 26:41.]