Why is the generic four-stage control cycle useful for understanding how to monitor and control projects?

One of the more difficult challenges of project control is finding a way to accurately measure progress.  The four-stage cycle breaks project down into specific goals that can be measured against the project baseline.  Deviations from the planned budget or time line can be identified and corrected swiftly.  The fact that it is a cycle, implying repetition of the process, demonstrates the constant need for project monitoring and control measures.  The final step in the cycle is to recycle the process resulting in continuous project control.