Thats why they suggest to hold because this is the one secret of trading if you want to get profit in trading and also relaxed your self when you comes to trade if you want gain profit to your invest.many investor they gain big profit in traders just waiting and be patient.
In the time of crash everyone have to wait only because no one know that what will happen to the market and the traders reduces in the number so the chances to get the profit becomes more lower. Holding at that time is beneficial and if someone will want to do trade then they have to trade only in times when the price will go better.