Comment by ebiester

The manager rarely needs to detect when the developer is that unproductive - it's glaringly obvious. In practice, even if you as the manager don't notice, the team will tell you that another developer isn't pulling their weight. However, you can't just fire someone in most organizations because their teammates have a gut feeling -- you need to have evidence as you get a case together. Further, some causes of poor performance are temporary; some are an indication of a lack of skill that can be remedied; some are that someone is a good worker in the wrong place in the organization; Sometimes, a developer is just lazy. A good manager will be able to discern this and determine if the solution is to change the environment in a way that brings success to the dev.

But if the case is that the developer needs to be "managed out," you need to build the case as a manager. This is where those metrics help.