Being stressed and flustered usually doesn't accomplish much. Over thinking, under producing, dredging up unnecessary conflict and angst.
Keeping cool prevents you from overreacting. You don't have to worry about flying off the handle or saying things that you don't mean.
Being calm allows you to see a situation crystal clear, without misunderstanding or jumping to conclusions.
By collecting your thoughts you gain insight into what is happening and what needs to be done to overcome it.
When you are calm, cool, and collected you are in control. You can navigate through change and challenge. You can overcome adversity and hardship. But even more than that you can get through it, unscathed and without regret.