“The first qualification of a general is a cool head,” Napoleon once said. So too for the Stoic. The student of Stoic philosophy learns many things but the first and the most important: Don’t make things worse by losing your cool.
Every situation is made better by a cool head. Even powerful people who know that anger is a powerful and effective tool will tell you that there is a big difference between deliberately expressing your frustrations (to make a point, to motivate someone, to defend yourself) and flying off the handle. Without the ability to recognize and direct your emotions, you become a slave to them.
If this is the only thing you take from Stoicism, even if you ignore all the other great teachings, this one is enough to keep challenging you for a lifetime. It’s got enough value in it to change you for a lifetime.