The speaker in this poem uses plural pronouns ("we," "us," and "our") instead of singular pronouns ("I," "me," or "my") because he is speaking not just for himself but for the entire group of soldiers experiencing the frigid cold as they remain trapped on the battlefield.

For example, when the speaker says "Our brains ache, in the merciless iced east winds that knive us," he means that it's not just his own brain that hurts: it's the brains of his comrades, too; and it's not just to him that the wind that feels like a knife: it feels that way to the others as well. 

By presenting the group of soldiers as a unified whole instead of speaking as an individual, the speaker of the poem emphasizes their communal sense of loyalty to one another as they endure the painful and horrifying exposure to the cold weather.

That sense of community, or comradeship, is important in this otherwise extremely bleak poem. The soldiers, as a group, are questioning why God is allowing them to suffer and die in this way, and the entire poem is about not only hopelessness but also the futility of suffering. Faced with this brutal situation, the speaker clings to the unity of his men and to his own sense of belonging to them by speaking in the plural.