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When you lose control of your asthma, the airways in your lungs become narrower. This is because the muscles in the airway have tightened; and/or the lining of the lungs has swollen; and/or the production of mucus inside the airways has increased. Inflammation makes the airways very sensitive and they tend to react strongly to things that a person is allergic to or finds irritating. When you maintain control over your asthma, the inflammation in your airways is reduced and your lungs become less sensitive to asthma triggers.

Think of it this way: we all know it is better for your skin not to get sunburnt, so we prevent it with sunscreen rather than dealing with sunburn after it has happened. Prevention helps the skin stay healthy. Similarly, good asthma control helps the lungs stay healthy.