Enjambment helps to pace the poem, pulling readers swiftly down the page in a way that might evoke the lure of the siren's song itself. Enjambment can also add moments of ambiguity and suspense.
The first line lines of the poem, for example, is enjambed:
This is the one song everyone
would like to learn: the song
Stretching this sentence across the linebreak introduces a little momentum and anticipation, as the reader can't know the relationship between this "song" and "everyone" until they continue on to the second line. The reader might expect the clause to end differently—"This is the one song everyone / knows," for example. Instead, it turns out that "nobody knows" it "because anyone who has heard it / is dead, and the others can't remember." Again, the use of enjambment here builds anticipation: the reader can't know what has happened to everyone "who has heard [the song]" until they've read past the line break.
Another striking enjambment comes between lines 24-25, when the speaker says,
you are unique
at last. Alas
The speaker enjambs a line across an entire stanza, adding extra emphasis as the reader lands on that ego-stroking "at last." The speaker seems to be saying not just that the listener is special, but that no one else has ever been special before—that finally, "at last," someone "unique" has come along. Once again, enjambment pulls the reader in only to abruptly pull the rug out from under them: no sooner does the reader feel "unique / at last" than the speaker makes it clear that they have fallen for the same "song" as all the others.