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.
Yes of course, basically everything that is done based on emotion will never be right, meaning usually the results will always be much worse because actions and decisions made based on emotion tend to be aggressive and far from your limits and that is the reason why the situation is usually much worse which in addition to losing more of your money will also increase the mental and psychological pressure of a gambler which is also what sometimes makes gamblers make crazy decisions like ending their lives or other decisions.
The point is, the ability to control oneself is indeed something very important that a gambler must have, as you said which does not only apply to gambling but also to various other things that happen in life.