What survival benefit does dissociation serve?

When placed under extremes of stress, humans tend to react with dissociation. This reaction can leave a person unable to grasp events unfolding around them and incapable of rationally responding to these events. Their ability to process information is diminished or completely shut down. Memory and reasoning become impaired if not completely disrupted.. Perhaps this protects the psyche when conditions are so stressful that it might otherwise be permanently damaged or destroyed, but doesn’t it put survival of the person at risk? Why do you think evolution led to the dissociative reaction to extremes of stress?