Sassoon fought in World War I, and most of his poetry from 1915 onwards is all about The Great War and the path of destruction it wrought. "Does it Matter?" takes us away from the battlefield and explores what life is like for the vets who bear lasting war-related injuries. More importantly, the poem mocks and criticizes a type of thinking that Sassoon felt was all too common when he was writing: the idea that fighting for one's country is noble, and that one can somehow overcome the emotional and physical trauma of battle by surrounding oneself with people who will pat you on the back and smile, instead of haul you to the nearest therapist.
This poem shows that the problem with war is that the patriots at home don't understand how serious it is. Their beliefs trivialize the loss of human life. The speaker, for example, repeatedly acts as though losing one's eyes and legs is no big deal (he even goes so far as to rhyme "legs" and "eggs," as if the two were the same thing).
"Does it Matter?" implies that people don't contemplate the horrors of war often enough; they hunt, eat, and generally don't pay enough attention to the soldier, which means that they're ignorant of all the physical and psychological damage it can inflict.