Adults faced with a situation in which death is a very real possibility frequently need and/or want to do whatever they can to make it easier for their survivors to continue living if they should die. Part of that process would be writing a will to designate how a person's possessions are to be distributed after that person's passing.
Adults in many parts of Europe (and Asia and Africa) during World War II were living under conditions in which imminent death was a real threat. Obviously, those directly involved in the military forces were at risk with every engagement. However, citizens were also endangered as fighting moved into populated areas and as aerial bombing became a more important part of the strategy. Under conditions in which many individuals felt unable to control what happened to their lives, one of the few things they could control was the preparations they could make for their survivors. Included in that process was writing their will.