Sometimes a single case study is persuasive because it provides for a clear example to follow. It also allows supporting mental image to be formed that can be both directional and motivational. I had told him the gory details two years earlier, soon after the crash. Perhaps my brief mentioning of that earlier incident provided Mike with an image that influenced his ultimate decision to use a bike helmet. Well, it's personal, and it's real.
Listeners can visualize themselves in a similar situation.
They can form a mental image of the event and relevant behavior. Later they can retrieve that image for personal direction and motivation. After all, it describes a real experience. And if the testimony includes an example of the safe behavior being advocated, the source of that testimony becomes credible as an advocate of the desired behavior. My bicycle story not only illustrated that dangerous bike crashes can happen and bike helmets prevent serious injury, it also made me credible as an advocate of bike helmet use.