Having rules to limit yourself in the markets is a must have by any trader who wants to be successful long term. Otherwise you're just a gambler or if you just buy and hold you're an investor. In an environment with endless opportunities to enrich yourself you can't go by ear, you need to have a system that says to you when the market is going against you and take the risk off before you make more damage to your account. Same with profits..
Moreover if you have rigid rules the emotions are unlikely to get a hold of you to the level where you can't make an informed decision.