If you have four L4 engineers on the team, all of whom are performing at the level described in the career profile as L5, but only budget to promote two of them, how do you pick which two? What if they have different managers, all of whom sincerely believe their report is the one delivering essential value?
If you have an organization with forced bucketing where X% of your team need to be given a subpar rating, how do you decide which one? If you don't have an obvious low performer you'd better have metrics.
This system is soul crushing but it exists all over the industry.