Well, one reason is that without metrics, prejudice and bias is much more likely to win the day. "I just don't feel like they're doing a good job" is affected b

Well, one reason is that without metrics, prejudice and bias is much more likely to win the day. "I just don't feel like they're doing a good job" is affected by all kinds of things unrelated to actual performance. At least having some metrics require you to think about whether that feeling is backed up by reality.