With more than a touch of withering irony, Sassoon sets out in particularly damning terms the effects of disabilities sustained during the First World War. The speaker sarcastically looks on the bright side concerning a variety of damaging disabilities: losing one's legs, blindness, and shell-shock.
In each case, he says that there are certain advantages to the respective condition. If you should lose your legs, then it doesn't really matter because there's still work that you can do, and people will always be kind to you. If you lose your sight, then that's no problem either because you can still sit on the terrace and turn your face to the light as you remember your experiences of war-time. And as for shell-shock, well even here there are certain benefits to be derived. You can get drunk to forget the horrors of the trenches and no one will think you mad.
In all of these cases the speaker subtly presents the dehumanizing, alienating effects of war. Yes, it's still possible to live without legs, eyes, or mental well-being. But it won't be much of a life; it'll be a sad, lonely existence in which your life-changing disabilities will isolate you from the rest of society.