People also are often unaware that the symptoms of asthma which cause the most concern (dyspnea, tightness, etc.) are less hypoxia and more hypercapnia. Hypoxia in normal people does not feel like breathlessness: it's lightheadedness, low energy, turning blue, etc. Asthma is almost definitionally airway hyper-responsiveness (and relevant structural changes): any fitness intervention that can help improve CO2 expulsion will probably greatly help an asthmatic seek relief. This is also why in acute attacks, pursed lip exhalations can really help.