A 38-year-old woman with moderate-to-severe persistent asthma is currently on inhaled fluticasone (500 μg twice daily) and albuterol as needed. Despite good inhaler technique, she continues to experience nocturnal symptoms 3–4 nights per week and morning peak flow variability of 35%. Spirometry shows FEV₁ 58% predicted. Her physician decides to add a long-acting β₂-agonist (LABA) to her regimen. Which of the following statements best explains the rationale for adding a LABA rather than increasing the inhaled corticosteroid dose alone?