War is the business of death, of killing. There's no doubt about that. While many of Sassoon's poems discuss actual death, "Does it Matter?" describes a less literal kind of death—the death of dreams, and the loss of a normal life. War is about more than snuffing out life on the battlefield—in this poem, it follows you home.
The poem argues that people can die before they really die; the soldier in this poem is, in many ways, already dead; he doesn't eat, he can't walk, and he can't see, just like a corpse.
Society itself is partly responsible for turning former soldiers into corpse-like figures; in the final stanza, for example, nobody worries about the soldier, which suggests that they treat him almost as if he doesn't exist.