Final answer:
A peak flow meter helps monitor daily changes in airway reactivity and lung function for a child with asthma.
Step-by-step explanation:
The nurse instructs a 10-year-old with asthma to use the peak flow meter because it monitors daily changes in airway reactivity. By measuring the peak expiratory flow rate, the meter helps the child keep track of their lung function and identify any changes or worsening of symptoms. This information can guide the child's asthma management plan and help them take appropriate actions, such as adjusting medication or seeking medical attention if necessary.