In one of those cases I was the bad manager. The data was a wakeup call that made me understand the magnitude of the problem.

In another case, I knew the manager well, having been on his team before. He was effective, empathetic, and inspirational. He was also overworked and perhaps a bit naively assumed good intent from everyone. The data let me explain to him that the coworker was not contributing and he had a real problem.