Then they seem kind of superfluous to 1:1s, right? Without 1:1s, performance reviews alone would not be very effective for employee growth. With 1:1s, they are more of a formality - not adding anything new to the conversation.

It's so the middle managers can see that the line managers have done some due diligence. This is to prevent the case where line managers "don't have a good feeling" about an employee but can't point to specific items that have been communicated to the employee. In an ideal world with great line managers you wouldn't need a performance review, it would be handled in 1:1s. That's not our world and we need rigid formal systems to increase the likelihood that basic steps are followed, on average across many managers and employees.

I don’t know… In my experience, managers’ bias can sneak into performance reviews. Sometimes that puts a spotlight on the manager and has consequences, but not always.