S/L (Stop Loss) should be used to anticipate wrong direction on forecasting. Suppose we did short on EURUSD but the market said the opposite (bullish), then S/L minimize our lost.
The problem is that mostly (including me) considered S/L was an obstacle to gain a hope for winning. Without S/L even though we saw less chance to get a winning, but we had a hope it would go back to the correct direction as we already predicted. But if it was failed, then big loss would be our great risk to blow our forex account.