During a rebuild the system is rewriting the replaced drive so that it’s contents agree with what the combination of the other drives plus the parity drive say it should be. UnRaid will detect the case of the disk drive reporting a write error, but what it will NOT detect is the case of the write appearing to be error free but a subsequent read returning the wrong data. In theory this should never happen, but it is always a possibility if the new drive is faulty in any way. Doing a parity check after the rebuild is just a confidence check that this (remote) possibility has not happened.