Emotional control is important in everything we do not just only In gambling but also in other areas of life. Emotional instability is a sign that you shouldn't be anywhere near gambling, if you can't control your actions you will end up having a lot of regrets. There are people who started out with gambling for fun but as time went on they got greedy and started gambling for profit, this lead to chasing their losses and also increasing their stake. The best way to control your emotions is to accept whatever outcomes you have, if you lose take a break and if you win also take a break, giving yourself a limit can help you keep yourself in control.
You are right, people should always learn to control their emotions because it's not just in gambling that they will have to control their emotions, their are other things things that they must not allow their emotions to take the deepest part of them. In gambling, lack of emotional control can only lead to more losses, at least I have experienced it and I know how it can be.