Realize that your lungs don't power themselves; you need to do breathing exercises to keep them healthy. The best way is to have strong lungs is to strengthen the main muscle (diaphragm) that makes them fill and empty with approximately 14 pounds of inhale and exhale pressure. 

Though it might be helpful if done exactly right, when breathing includes intercostals and core/AB muscles, it can possibly cause tensions and restrict deepest easiest breathing. Another factor is to release the tension caused by that poor rib/AB breathing pattern allowing the entire rib cage to open up wider for a deeper easier inbreath.

Why do breathing exercises?

Doing breathing exercises can help minimize the risk factors for most any lung disease and the possibility of it going from mild to moderate or severe and deadly.

Doing breathing exercises can help you recover from or reduce infestation from COVID-19.

Doing breathing exercises can help you manage the inflammation caused by stress.

Doing breathing exercises can help minimize the risk factors for COVID such as the risk factors for increased high blood pressure. There are numerous sound scientific studies that document that blood pressure can be lowered/controlled with proper daily breathing exercises. Having strong breathing is something you can measure and practice.

Doing breathing exercises, apart from cardiovascular exercises is vital, since cardio works your heart, not your breathing muscles as specifically or as heart.  Plus many cardio exercises invite  restricted breathing such as with exercise induced asthma.

Having strong breathing capacity and balance is an important factor in longevity especially since your breathing starts to decline in your 20s unless you are doing something to maintain it.

People with respiratory illnesses like COPD are more at risk with COVID 19. COPD is the 4th leading cause of death in the US. Breathing exercises, particularly those that strengthen the exhale are something that pulmonary rehab focuses on “to combat overinflation, the breathlessness and fatigue that accompanies it”; However, you can do these on your own as part of prevention and good health practices to help with a strong respiratory system.

Doing breathing exercises can help you recover from COVID-19.

Maintaining oxygen levels can be helped through mechanically sound breathing. Having a balance of strong inhales and exhales “oxygen and CO2” is what makes for good scores.

COVID-19 progresses to pneumonia. 
It's critical to have an efficient cleansing cough. Start by simply focusing on your exhale and making it as deep as you can while pushing out your belly. This hopefully will trigger a cough. I call it squeezing the sponge. Remember, the densest most oxygen reached deep part of your lungs is in the middle of your body, not at the top. So the belly push- out and deepest exhale help dislodge a build up of mucous, push it into your bronchioles, they do not like that and you cough. It is a good thing and considered productive, wet or dry.

Shallow breathing results in a vicious loop of stress and tension. 
NOTE the Optimal Breathing Window to get more reality on that. While the easiest way to learn to breathe, snoring and apnea excepted, is while you are on your back, breathing on your side does allow for the lateral expansion of the lower part of the lungs. 
We have several exercises in our Energetic Calm exercises that show you how to do that.

Doing breathing exercises can help lower the inflammation caused by stress and high cortisol.

Breathing is the mind body connection. If you are tense, it's likely your breathing is telling your mind that the environment is dangerous or over exciting. Stress makes for a higher heart rate, higher cortisol, and a lowered immune system.

Stimulating the vagus nerve In order to achieve a more parasympathetic rest and digest state is achieved through the breath via our reflex triggering exercises C21,22,23,24,25 both in the location of the breath and in the amount of expansion. An abdominal thoracic breath “belly and ribs” that has good range of motion front, side and back, widens a narrow exhale giving you more choices as to the pacing and slowing down of the breath. But there are many more exercises that replace or augment that.

Stress makes us brace, bracing makes us feel prepared but we pay serous dues for bracing too often as that invites almost permanent shallow breathing. We humans are very resilient, but we need a reset from time to time. In order to reboot and release we need to take a few deep easy belly breaths through our nose and NOT exhale out our mouth as is often erroneously taught. Many are not familiar with this and overbreathe causing increased anxiety. 

Done properly, this simple interruption of stress gets you out of the fight or flight for a few seconds and this is what your body needs to reset and keep going. The C21,22,23,24,25 reflex triggering exercises plus a few others in our Calming, relaxing online program are based on the absolute fundamental/foundation of healthy deepest breathing. After 40 years of studying breathing, I do not know of any others that do this as well.