Following a cease-fire, and especially in the absence of a peace agreement, prisoners of war from the opposing forces may be held in camps with no prospect of release (in explicit contravention of the Geneva Convention). Soldiers are kept apart from their families, whom they may not have seen since the war began. The prisoners effectively become hostages held against the day when peace negotiations commence. Furthermore, unless the prisoners are registered with the Red Cross (which may be obstructed), there is no way in which their families can no that they are still alive.
As an example, a year after the Gulf War ended in 1989, from 69,000 to 109,000 Iranian and Iraqi prisoners of war remained in camps with no prospect of release.