It restores balance, and it helps build up and conserve bodily energy. Heart is slowed Pupils return to normal size Blood pressure drops etc. And, the Parasympathetic responds much more slowly than Sympathetic branch. Parasympathetic rebound - parasympathetic branch overreacts during intense fear. It can cause death.
Some soldiers literally die of fear in combat.
Older persons or those with heart problems may have a heart attack or collapse due to sympathetic activation. Accuracy is doubtful. Often a serious invasion of privacy. All it really does is record general emotional arousal. It can't tell the difference between lying and fear, anxiety, or excitement.