Employees spend effort on various activities most of which are productive and desirable and some of which are unproductive and undesirable.
It is also common fact that developers spend more time in coding than in other significant areas like requirements, design and testing.
This data when used in conjunction with defect metrics can throw significant light on areas that need more focus.
For example, if defect metrics show more defects in requirements area, and effort metrics show that relatively less effort is spent in requirements analysis – it becomes obvious that the organization has not been focusing on requirements analysis as much as it should and that balance is out of order.
Then the course of action is clear – spend more effort on requirements analysis and restore balance.