A 70-year-old woman with known idiopathic pulmonary fibrosis is admitted to ICU with acute hypoxaemic respiratory failure. ABG on 15 L/min reservoir mask: pH 7.44, PaO2 6.5 kPa, PaCO2 3.8 kPa, HCO3 22 mmol/L. SpO2 82%. She is tachypnoeic (RR 38) and distressed. The ICU team considers high-flow nasal oxygen (HFNO). What is the primary physiological benefit of HFNO in this scenario?