One of the best advantages of poetry over any kind of prose is its conciseness. Because all the excess words have been squeezed out, the message left on the page is so much more concentrated. Any imagery or emotional effect that the author wants readers to respond to is right there, standing out in a very visual format, not buried in a chapter somewhere. Also, by its very nature poetry is designed to draw emotional responses out of us. We expect the author to give us vivid imagery, to put us through emotional experiences that we either relate to in our own way, or experience vicariously through the author’s words.
Because Siegfried Sassoon was writing about his very graphic, emotional, and often controversial experiences in and beliefs about World War I, perhaps he felt that the world would accept his autobiographical account of it in poetry form better. Such a deeply passionate experience as fighting in and surviving a world war seems fit for the poetry genre by its very nature. For example, in Counter Attack, he describes how the living, with gangrene legs, step over the rotting bodies of the dead. They are "face downward, in the sucking mud, / And naked sodden buttocks, mats of hair, / Bulged, clotted heads slept in the plastering slime." Readers know this is a true account of Siegfried Sassoon's experiences. This vivid description provokes a strong emotional response in readers.
Also consider that politically, Sassoon wanted to gain an audience to listen to him, and although he did eventually write three autobiographical novels, his first instinct while recovering from his war injuries was to employ his best writing skill--poetry. Early on, many of his British contemporaries shied away from what they felt was his unpatriotic view of the war. Sassoon may have felt that he would not have drawn a large audience with an entire book of his shockingly graphic accounts of trench life, of the horror and pain, especially given his often satiric voice about war. But a poem? It is shorter, more accessible. He likely did pick up more readers through that medium. He could use all the figurative language techniques to present the brutality of what he and other soldiers experienced. He could use symbolism and imagery to create compassion in his readers, to help them feel as though they were there, watching their friends die with him. In this manner, Sassoon hoped his poems would cause others to question their political and military leaders’ motives in war.