The formal practice of mindfulness makes it much easier to find your centre and feeling of balance in a stressful situation. This will happen because you will practice finding your centre during a formal practice. If you have a consistent mindfulness formal practice, you will have lots of opportunity to experience feeling off balance. This happens as our attempts to be mindful often present a myriad of distractions that take us away from the present moment and contribute to feelings of being off center. Then, we have an opportunity in the formal practice to learn to handle these reactions in a way that promotes feeling balanced and centered. Subsequently, when we find ourselves in a demanding situation, you can apply what you have learned to the actuality of the present moment in which you find yourself.
A further reason for the need for formal practice is its power to stimulate the para-sympathetic nervous system. This in itself promotes a strengthening of this important nervous system response that leads to feelings of calmness and well-being. You will not only become more familiar with this feeling, but, it will also give you clues how to activate it when the powerful responses of the sympathetic nervous system are triggered in a stressful situation.
Finally, the experience of stress is often triggered by habitual and conditioned ways of perceiving and reacting to the various demands in your life. By definition, habits and conditioning are very strong and ingrained and, for this reason, mindfulness is a power antidote to bring to these forces in such a way that they become much less impactful.