Why track actual costs and resource usage on projects?

The importance of tracking actual costs and resource usage in projects depends upon the project situation. For some projects, tracking actuals is unnecessary or is not worth the effort required. In other cases, however, tracking actual costs and resource usage is an essential aspect of the project control function. In such cases, a system must be put into place to support the tracking process, and the collection/recording of the potentially voluminous quantity of data requires strong organizational discipline. Why then is tracking actual costs and resource usage on a project ever worth the effort required to accomplish it?