For example, if we have a RAID 5 with a defective HDD, the cannot tolerate another HDD failure, otherwise, the data will be lost.
But if there's a spare drive, this one will immediately replace the defective HDD. The missing data is rebuilt on the spare drive and as soon as it's over, the RAID system is no longer in failure mode.
So the spare drive is never used unless there's a HDD failure.
Only the following RAID systems can handle a spare drive :