A case study is a detailed description of one person, family or group, used in research or training to show how a condition presents or how a treatment went.
Case studies record detail that larger studies can't. One person's history, their assessment and what happened next. That makes them useful for describing presentations that are rare, or that larger studies were not designed to capture.
The limit is that one person's story can't tell you how common something is, or whether a treatment works for most people. For that you need larger studies, such as a randomised controlled trial.
Published case studies remove names and identifying detail. When you read one, treat it as a description of what can happen, not a prediction of what will happen to you.