"William Street" uses alliteration to bring its evocative imagery to life on the page. Alliteration—along with the related devices consonance, sibilance, and assonance—makes the sights and the smells of William street more vivid and intense.
Listen to the alliteration in the first stanza, for example:
The red globe of light, the liquor green,
spilt on the stones, go deeper than a stream;
The flurry of repeated sounds evokes the chaotic, swirling mass of lights. Shared sounds also create the impression that all these lights are bleeding together, as in long-exposure photography.
The alliteration gets even louder in the third stanza:
Smells rich and rasping, smoke and fat and fish
and puffs of paraffin that crimp the nose,
The rush of sounds conveys just how overwhelming the smells of the street are. The consonance and sibilance within words add to the effect ("rasping," "puffs of paraffin," "crimp").
In the poem's final stanza, /h/ alliteration of "hunger at their heels" draws readers' attention to the fact that the people of William Street are engaged in a constant struggle to survive. The breathiness of that /h/ sounds even suggests the breathlessness with which these "dips and molls" try to outrun their hunger. These people roam "the pavements of their pasturage," the speaker says, alliteration again adding emphasis to an important image in the poem: the urban jungle as a "pasturage," or grazing ground, for society's less fortunate.