Hot spares are a good addition to a fault-tolerant array.
They greatly reduce the impact of monitoring on the array reliability.
If a drive has failed in a fault-tolerant (RAID 1, RAID 10, RAID 5, or RAID 6) array, a vulnerability window opens.
If another drive fails during this vulnerability window, the data is lost.
Without the hot spare, the rebuild does not commence until someone walks in and physically replaces the failed drive.
Hot spare drives allow the controller to commence the rebuild immediately, without having to wait for administrator intervention,
thereby reducing the vulnerability window.
The need for a hot spare increases as the number of disks in array increases.
Hot spares are most effective when a single hot spare drive is shared between several arrays.
Consider for example an 8-bay NAS.
If there is only one RAID 5 array in the NAS, then RAID 6 may be a better option than a hot spare.
The hot spare drive just sits there idly; in a RAID 6 array, the same drive would be utilized to improve a read speed.
However if you need two RAID 5 arrays, the hot spare drive is shared between these two arrays,
reducing the total disk space overhead.