The main idea is that you want to cut your losses, especially if you bought at the wrong time.
Generally, it is better to not enter stop loss orders. Rather, watch your stocks closely and know ahead of time the exact price at which you will immediately sell to cut a loss. A stop loss may be helpful if you can't watch your stocks closely or if you're indecisive.
To learn more about selling rules please visit IBD University at this link - When to Sell Stocks