Confidentiality, integrity and availability, also known as the CIA triad, is a model designed to guide policies for information security within an organization. These topics give many, many reasons for making health information secure. HIPPA guidelines include specific policy and practices in each of these areas.
Personally, I see the biggest risk in the Integrity area. If my doctor, dentist, hospital, etc. is not looking at correct information, I may be given the wrong medicine, have the wrong tooth pulled, or much worse. Years ago, as the manager of a large government computer facility, I had to say "get your own computer" when the health office requested to use our large minicomputer to modernize their operation. We had operators, programmers, etc. with full privileges, not only to view, but to change anything on that computer. Now, it may or may not be obvious that your personally identifiable information (like what diseases you have) has been changed, but imagine temporarily changing it just before the doctor looks at it and then changing it back afterwards. Today, it is not only a doctor processing this information, it is medical robots and pulse-monitoring machines and pacemakers and ... Well, the information must be accurate.
For example, in the current political campaign, there have been questions, reports, and interest in the age and health of both Clinton and Trump. It is devastating when incorrect information is publicized.