Why "just breathe"? ...because instantly taking a deep breath (or two or maybe even three, at times) makes a person feel better.  Deep breathing is a natural stress reliever, sending signals to our brain to be calm.  With deep breaths, we can decrease our blood pressure and heart rate.  During massage sessions, it is important to monitor a client's breathing for clues, and to allow them to breathe into deeper pressure techniques.
  Often, it gives just that extra moment of pause when we are upset or frustrated; even stopping us from saying something we may be sorry for later.  Sometimes to take in an aroma that we love and recall later. Remember the smell of the fresh salt air or the scent of your favorite flower, Thanksgiving turkey or homemade cookies and pies.  Obviously, this list could go on and on.
  Ultimately, because it just feels good.