“Bent double, like old beggars” This simile puts an image of broken human beings in our mind and gives us sympathy towards them and their circumstances. We have sympathy as we would never like to be placed in the situations which these soldiers faced.
“Gas! GAS! Quick, boys!” This command tells us there is danger coming. It wants us to feel the danger and get us more involved in the text.
“Obscene as cancer, bitter as the cud.” This over exaggeration is used by the writer to make sure the ready knows just how grim it was.