Snow, a beautiful natural phenomenon with many rich metaphorical associations, has been used to great effect by many poets. Looking at a few of these can help us understand some of the background for Macleish's use of snow. In Emily Dickinson's, "It sifts from Leaden sieves -" (available here), written in c. 1862, Dickinson never actually uses the word snow. However, based on earlier versions of the poem, it's now assumed that the "It" Dickinson refers to is snow. Unlike MacLeish's mention of the snowflake in the title of his poem, Dickinson's snow comprises the whole poem's subject and does not seem to be an obvious metaphor for another phenomenon. She writes beautifully and euphonically about the effects of snow on city and natural terrain. Much of the power of the poem comes from Dickinson's choice to personify the snow—rendering its appearance and its movements soft and sublime.
In Robert Frost's famous 1923 poem "Stopping by Woods on a Snowy Evening" (available here), Frost uses snow to evoke a visual manifestation of a quiet moment of pause and pensive solitude. For some, Frost's poem is an allegory for life in the face of mortality. In this interpretation, the speaker's rest in the woods is a moment to take stock of his passage through life thus far, as well as the path that lies ahead. The peaceful beauty of the scene alongside the darkness and cold of the speaker's surroundings can be compared to the poignancy of life's beauty as one approaches death. The speaker's horse shakes its bells "To ask if there is some mistake"—a sentiment one could argue is felt universally, when one ponders the circumstances of human life and its transience.
In "Munich Mannequins" (available here), Sylvia Plath uses snow in a drastically different way than Dickinson, Frost or MacLeish. In "Munich Mannequins," written in 1963, snow represents the misogynistic "perfection" demanded of women by society, which in some cases, is a death sentence. She writes that "Cold as snow breath, it tamps the womb"—killing the vitality, enterprise and fertility of women. Given that the poem is about mannequins, and likely, unattainable beauty standards, one could posit that Plath is concerned with the physical ramifications of the patriarchy as much as the symbolic.
