What exactly was the point of Odysseus seeking out Tiresias/the Prophet?

In the original myth, Tiresias informs Odysseus that blinding Polyphemus has invoked Poseidon’s wrath (he did not know this previously). He predicts Odysseus will return home and travel to a distant land to appease Poseidon and also warns him not to hurt the sacred cattle on the sun god’s island.

However, I am confused what was the benefit of speaking to Tiresias in the musical. Odysseus already knows Poseidon is after him, he figures out himself they shouldn’t kill the cattle, and he even figures out the way home himself from the sirens (he does that himself in the myth too if I remember).

Tiresias makes vague references to future events in the song, but I didn’t get any indication they were very helpful to Odysseus. It seemed to me that other than seeing his fallen friends and family, the underworld trip was a bit of a detour for no reason. Is there subtext I missed?

As a musical choice, I think I understand why this was done because having the song spoil specific events in an obvious way would have been kinda lame, even if you already know the myth, but still?