Projects never go bad overnight. It takes time. They slowly drift away from the baseline. Maybe it is a change request that goes undocumented, or a series of tasks that run a little late, or an over-optimistic employee not realizing they are in trouble, or misinterpreted communication. They all add up over time and are very difficult to detect while in the heat of the delivery.
It often takes an experienced outsider who is removed from the history and politics to see the issues and make the recommendations that will keep a project on track. The Project Health Check provides the data and accountability you need to maintain your project on a proper course.