"Work" is overflowing with assonance, which—along with the related devices of alliteration and consonance—calls attention to important moments and generally ramps up the intensity of the poem's language.
In fact, these sonic devices often overlap. Take line 3, where there's /oo/ assonance, /s/ alliteration, and /p/ consonance:
One small mouth, a soup-filled spoon,
The sounds of the line blend together, evoking the simplicity and ease of the woman's life when she was caring for a single child.
As the woman's work intensifies, however, so do the poem's sounds. Listen to lines 7-11, for instance:
[...] gathered barley, wheat, corn.
Twins were born. To feed four,
she grafted harder, second job in the alehouse,
food in the larder, food on the table,
she was game, able. [...]
Though the poem doesn't follow a steady rhyme scheme, the combination of assonance and consonance creates many full or slant internal rhymes. Here, note the full rhymes between "corn" and "born," "harder" and "larder," and "table" and "able," as well as a slant end rhyme between "corn" and "four."
Again, these emphatic sounds seem to multiply along with the woman's children. In lines 22-23, listen to the shared long /ay/ sounds of "trains," "came," and "planes" (with "trains" and "planes" again creating an internal rhyme). There's then another slant end rhyme between "sound" and "now" in lines 23-24.
The sounds only grow more intense in stanzas 7, where incessant long /ee/ assonance ("TVs, / designed PCs, ripped CDs, burned DVDs) evokes the endless nature of the woman's work. And this sound continues into stanza 8:
she trawled the seas, hoovered fish, felled trees,
grazed beef, sold cheap fast food, put in
a 90-hour week. [...]
Note, too, the /f/ alliteration and consonance here ("fish," "felled," "fast food," "beef"), as well as the internal rhyme of "seas" and "trees." Overall, these sounds feel relentless, thereby suggesting humanity's seemingly unstoppable greed and exploitation of the earth.
In the last stanza, /ay/ assonance ("lay in a grave"), /s/ alliteration ("scattered," "swam," etc.), /w/ alliteration ("world, wept") and an internal rhyme between "rain" and "grain" adds intensity to the imagery of the woman sacrificing herself for her children.