The alliteration in "Marrysong" makes the poem sound unified and harmonious. The most prominent alliterative sound in the poem is the /w/ sound, which the speaker stretches across a number of lines. Consider, for example, the way the /w/ sound appears in lines 4 through 9 (including in the "kwa" of "quarried"):
in the walled anger of her quarried hurt
on turning, see cool water laughing where
the day before there were stones in her voice.
He charted. She made wilderness again.
Roads disappeared. The map was never true.
Wind [...]
The alliterative /w/ sound appears at least once in all the above lines. And even though the /w/ doesn't always dominate the overall sound of a given line, the frequency with which it occurs in this section ensures that readers will pick up on its wobbly effect. In turn, these lines have a certain topsy-turvy quality, as the repeated /w/ creates a slight sense of motion and instability—an effect that captures the husband's inability to keep up with the wife's constant changes.
In other sections, the speaker uses alliteration to enhance the musicality of more isolated moments. For instance, the speaker alliterates the /j/ sound in line 14:
new country at each jaunty helpless journey.
This is just a brief instance of alliteration, but because the /j/ sound is very noticeable, it goes a long way in making the line sound distinct and cohesive. Similarly, line 12 features the /sh/ sound in a quick, passing way that still defines the line's overall feel:
the shadows of her love shortened or grew
The quick repetition of this alliterative /sh/ sound lends the line a swishing sensation that reflects the wife's sudden and unexpected emotional vacillations. In turn, alliteration helps the speaker not only bolster the poem's musicality, but also subtly imbue the language with a feeling of fluctuation and movement that mirrors the wife's emotional state, which is constantly in flux.