The alliteration in "Disabled" adds emphasis to the language, calling attention to certain words and often linking them together to suggest some kind of connection. Take, for instance, the alliteration of the /w/ sound in line 1:
He sat in a wheeled chair, waiting for dark.
This spotlights both "wheeled" and "waiting," subtly linking the man's wheelchair (and, thus, the injury that put him in that wheelchair) to stillness and "waiting" around. The poem implies that he has lost his independence, so things only happen to him. Most of his time, then, is spent sitting in his chair trying to tolerate the monotony of doing nothing.
In line 2, the alliterative /g/ sound is quite noticeable:
And shivered in his ghastly suit of grey,
This adds intensity to the language, underlining the idea that the soldier dislikes his hospital-issued "suit," which he thinks is "ghastly." In this way, this moment of alliteration helps readers sense the young man's frustration and unhappiness with his current circumstances.
The sibilance in line 33 in the phrase "smart salutes" helps highlight the soldier's romanticized ideas about going to war. The hissing /s/ draws attention to the image of a disciplined and good-looking soldier giving a superior a strong, sharp salute—and this, in turn, hints that the young man only really thought about how he would look as a soldier, not necessarily what it would feel like to be on the battlefield.