Nowadays, there are plenty of books, articles, and websites affirming the benefits of mindfulness practice in coping with the stressful, and overwhelming aspects of busy modern life. Like many relaxation techniques, it can be useful for getting into a reflective, big-picture, solution-finding, accepting state of mind and dealing more effectively with day-to-day living.
In therapy, it is useful to develop our capacity to observe but not be overwhelmed by our experience, be that thoughts, feelings or bodily sensations (for example clenched fists, butterflies in your stomach, or hunched and tense shoulders). This capacity to observe is like building specific muscles at the gym - it doesn't happen straightaway, but benefits accrue from consistent practice.
Mindfulness in therapy is finding a balance between being physically relaxed on the one hand, and paying close and careful attention to experience on the other. Your therapist is there with you in this relaxed observing so you can notice what's going on and not be all alone with it. With acceptance and compassion you observe from some distance. This means the raw data is being clearly seen, without being censored it or filtering it out.
This experience can be restful and helps gain peace of mind. It can also help to listen to yourself - your real self. It is invaluable to escape all the passing influences and reconnect with one's true thoughts and feelings.
This approach is very useful for the treatment of depression, anxiety, trauma, anger, relationships or any other issue where it is useful to reduce symptoms so the issues can be clearly seen and understood. The techniques used in the therapy room are also useful in everyday life when confronted by stressful situations or emotional triggers.
Knowing or understanding yourself makes a whole lot of the stresses and issues we face easier. Old outdated ideas about ourselves and our world can be reviewed; we can be more flexible in our thinking, more relaxed in our attitudes. Mindfulness in a therapy session can be relaxing but more importantly help inform us about who we are and how we might become our better selves.