Breathing exercise is the same for the mind as asana is for the body. Yoga poses transform the body and pranayama (breathing techniques) transform the mind. The former develops your sense of balance, strength, posture, the latter your mental abilities. There is a connection between the breath and the functioning of the mind. If you are upset you have different breathing than when you are in love or afraid. Your state of consciousness changes, the quality and duration of your breath changes. Observe your breathing every day!
Breathing exercises, even the simplest ones, develop qualities of mind. Some increase the ability to concentrate, others make you calmer. The mind works better, so you live better in the world. When you are in the flowing state, life flows through you.