Like most other insurances, Private Medical Insurance is designed to return you to the state you were in prior to needing to make a claim i.e. to cure you of your medical ailment leading to a claim. Such claims could include heart attacks and heart bypasses, ligament tear, back pain, shoulder rotator cuff repairs and many more.
In order to cure such ailments, one would need to see specialists, undergo test and pathology, receive scans and imaging such as MRI or CT and possibly undergo surgery in a private hospital. As you can see, there are lots that can be covered under your PMI policy.
Unfortunately, many medical issues have no known cure e.g. Alzheimer, Diabetes, Asthma and Irritable Bowel Syndrome. Such conditions are known as chronic and non-curable. Whilst the insurer will pay all costs associated up to point of diagnosis of a Chronic condition, payment will more than likely cease once a Chronic diagnosis has been confirmed by your Specialist Physician.