The poem uses anaphora often to emphasize its thematic ideas. For example, note the repetition of "Who"/"Whose" in the first stanza:
Who made him dead to rapture and despair,
Who loosened and let down this brutal jaw?
Whose was the hand that slanted back this brow?
Whose breath blew out the light within this brain?
These questions are rhetorical, and the speaker knows exactly who this "Who" is: the ruling class. The use of "Who"/"Whose" is a way to repeatedly call out this class, perhaps until the rulers own up to their sin. Think of the way a teacher might scold a misbehaving student by saying, "You know who you are."
The repetition of "To" at the starts of lines 12-14 reflect the breadth of God's power and vision for humanity. Each "To" lays out another thing that human beings are meant to do, and each "To" also is something that that laborer is unable to do—thereby denying God's will.
Later, the speaker uses anaphora of "More" in lines 19-21 to illustrate the worker's miserable condition, piling on the things that pale in comparison. Anaphora amplifies the sense of incomparable misery while also adding to its meaning. There is nothing "more" revealing of greed, nothing "more" ominous for the state of the human soul, and nothing "more" threatening to the universe, than the exploitation of the worker.
The poem's final stanza uses anaphora to again pose a question repeatedly, heightening the effect of Markham's appeal to the moral conscience of the poem's reader. Lines 44, 45, and 47 all ask "How" history will look back on the state of the exploited worker. Through the emphasis of repetition, these questions suggest that society will fail to change in the ways the poem calls for. The phrases "How will" and "How answer" point to a future in which society's response to the crisis of labor exploitation is very much in doubt.