Production readiness reviews (PRRs) are intended to ensure that enhancements merging into Kubernetes are observable, scalable and supportable, can be safely operated in production environments, and can be disabled or rolled back in the event they cause increased failures in production.
While approvals for an enhancement are still required from the respective SIG, production readiness reviews are done by the Production Readiness subproject of SIG Architecture.
It is useful to have the viewpoint of a team that is not as familiar with the intimate details of the SIG, but is familiar with Kubernetes and with operating Kubernetes in production.
Experience through our dry runs in the Kubernetes 1.17 - 1.20 release cycles has shown that this slightly "outsider" view helps identify otherwise missed items.
Full documentation on the production readiness review process can be found on the SIG Architecture page here.