Whether extra disks should be live or spare now depends on HDs versus SSDs
Suppose, not entirely hypothetically, that you have a server with at least three or four spare drive bays and you want to build a mirrored storage setup that can maintain redundancy without requiring an in-person drive swap should a drive fail. Let's say you go with three drives in the system in total. Obviously two of them have to be mirrored in order to get your basic redundancy, but the third one could be used either to make a three way mirror or held back as a (configured) hot spare.
(If you can use four drives, you can have a three way mirror and a spare, or a four way mirror.)
In the old days of hard drives, you generally might as well use your extra drive as an additional mirror instead of holding it back as a spare. Either way the drive was usually going to be powered on and spinning, and this was enough to start counting down its lifetime. Having it active as a mirror got you some additional read IOs a second and meant you kept all your redundancy without needing any time for a mirror re-synchronization after a drive failed.
(The folklore, at least, was that powered on time was the most important thing for hard drive lifetime because of wear on the main motor. There were other mechanical parts involved in things like the read/write heads, but my impression is that they were usually not seen as a likely source of failure. This may be incorrect in practice.)
In the new days of solid state disks, I've recently realized that that's no longer really true. The amount of time a SSD spends powered on does matter, but so does the amount of data that's been written to it (and perhaps the read volume as well). By keeping your extra SSD out of the mirror, you avoid exposing it to all that write traffic, which prolongs its lifetime. By putting less load on your extra SSD in general, you also hopefully make it less likely that the SSD will suffer an infant mortality death too close to another SSD dying. Or in short, with SSDs, write and read activity now matters. A quiet drive is likely to be a longer-lived drive, so keeping your extra drive quiet is good.
(At the same time you want to poke the extra drive periodically just to make sure it still works. Regular SMART probes might be good enough, but for caution you might want to do a tiny bit of writes every so often or something.)