Suffering is paramount in The Plague. We watch how characters react to, cope with, and learn from hardship. This last one is particularly important; one character goes so far as to cite suffering as his greatest teacher. So what do we learn, exactly, besides the helpful tidbit that we should always have anti-plague serum lying around? That the world is indifferent to the ceaseless and irrational suffering of man. The solution presented by The Plague is that, even knowing that it is a losing battle, we must fight against suffering, fight on the side of mankind.
The citizens of Oran suffer as much before and after the plague as they do during it; the only difference is that they are aware of their suffering during the outbreak.
While suffering can not ever be justified in The Plague, it can still be beneficial.