Resistance to inhalation (breathing in) strengthens the diaphragm, the major muscle of breathing, and the accessory muscles of the neck. Resistance to exhalation (breathing out) develops strength and tone in the abdominal muscles. Additionally, exhalation resistance mimics pursed lip breathing and helps to keep the airways clear of mucus.
An incentive Spirometer does remind you to take in a deep breath and some even use it as a tool to measure their gross lung volumes.
However, while Incentive Spirometry encourages deep breathing, there is no resistance to the intake of air through the device, so it does not strengthen the respiratory muscles.