Apple - Pros and cons of enabling “Put hard disks to sleep when possible?”
on older traditional rotating disk drives it will save some power
on the same type of drive it often parks the heads making it less fragile in cases of unplanned moving/shock/drop
rotating hard disk drives will live shorter due to the fact that it’s the starting and stopping that kills them, not running idle
slower initial access to the disk as it starts after it was put in standby (it needs to spin up before it can be used)
None of this really matters to solid state disks as they are pure electronical devices with no mechanics. Their power use is directly proportional to disk activity. While they often do have extra low power modes, you won’t actually benefit much from it since it is not much less than its normal power regulation.
The reason this is plural is because it can also apply to external disks, and desktop macs used to have space for up to six harddisks. The 2009 Mac Pro has 4, for example.