[ebiester]

Because the alternative is based on your manager's impressions. You get fired because "your manager doesn't like you." You get a promotion because "your manager likes you." Your coworker sits around doing very little and nobody notices for a few months.

Your company and boss is sued because someone says their firing was based on discrimination because they can't prove it was for performance.

The truth is that managers can suss out 90% of the problems without a number. However, we are asked to document the hell out of it if we want to do something about it. And twice in my career I've been wrong: I mistook someone who was quiet for not doing much until I dove into the work. I trust my gut, but I confirm with numbers.