How does incentive spirometry prevent fever and why is it so importanr?

Prevents infection.. When a person doesn't take deep breaths, the airways not being used tend to close off so that the inhaled air is routed to the lung space that is being used. But, secretions can collect in these airways and stay there where they can become infected, leading to acute bronchitis, pneumonia, etc. Incentive spirometry causes you to open these airways, expectorate secretions and prevent infections!
Not related . Incentive spirometry has no relation to fever. It helps improve lung function and may help prevent post op pneumonia.