"How you can sit there, calmly eating muffins when we are in this horrible trouble, I can't make out. You seem to me to be perfectly heartless." "Well, I can't eat muffins in an agitated manner. The butter would probably get on my cuffs. One should always eat muffins quite calmly. It is the only way to eat them." "I say it's perfectly heartless your eating muffins at all, under the circumstances." --
Thus Gotama [Buddha] walked toward the town to gather alms, and the two samanas recognized him solely by the perfection of his repose, by the calmness of his figure, in which there was no trace of seeking, desiring, imitating, or striving, only light and peace
Concentration is a cornerstone of mindfulness practice. Your mindfulness will only be as robust as the capacity of your mind to be calm and stable. Without calmness, the mirror of mindfulness will have an agitated and choppy surface and will not be able to reflect things with any accuracy.