The Semi-Fowler's position, where the head of the bed is elevated to a 30-45 degree angle, is primarily used to improve respiratory function and aid in drainage. It's beneficial for patients with respiratory conditions like COPD or pneumonia, as it helps to reduce congestion and improve breathing. Additionally, it's used during feeding with a nasogastric tube to minimize the risk of aspiration and for individuals with GERD to prevent stomach acid reflux.