A middle-aged woman told her physician, "I've never had a chronic illness and don't think I can develop one." "That's a good delusion. Keep it," the doctor replied. Yet delusions have a bad history, being associated in many minds with horrific scenes from countless horror movies. But delusional thinking can have benefits too.
A person whose life has been a succession of personal disasters from psychiatric hospitalizations to painful injury to poverty develops the delusion that the FBI is watching them. Similarly, a soldier on the battlefield, close by comrades who are being killed, tells themself they will survive. In the first example, the delusion may protect the person from suicide, since if the FBI is watching them, they must be of great importance and is thus worthy of respect. A soldier's delusion of certain longevity keeps them fighting and increases their and their comrades' probability of survival.
Yet delusional thinking is, of course, generally destructive since only through reality-based thinking can the problems of living be resolved. The anxious depression that accompanies delusional thinking is proper since this type of thinking reduces situational adaptability and thus increases personal peril.