I have a lot of clients ask me this and I even remember thinking when I was a beginner that it really wasn't important. I couldn't have been more wrong. More and more research keeps coming in about the many benefits of deep breathing with the diaphram. It has been proven to reduce blood pressure, reduce stress, work the core more, give much needed oxegen to the muscles, reduce soreness, and makes exercise more doable and enjoyable. When lifting a heavy weight, that is the time to exhale either through the nose or mouth. When releasing the weight that is when you inhale deeply. You should see your stomach rising when inhaling. Even runners can benefit by inhaling every two steps and exhaling on the third step. Try it and see if you are able to lift more and enjoy your workouts more!