It does though, it defines when to complete the write command to a client and when general any IO should be allowed on an object (both need #copies >= min-size), from the Quincy PG settings docs:

  > Sets the minimum number of written replicas for objects in the pool in order
  > to acknowledge an I/O operation to the client. If minimum is not met, Ceph
  > will not acknowledge the I/O to the client, which may result in data loss.
  > This setting ensures a minimum number of replicas when operating in degraded
  > mode.