Many people leave their computers on around the clock. This usually implies that all the attached hard disks are always spinning. Constantly spinning up a hard disk normally increases the chances of drive failure. When a disk is not powered it should last longer than if it was spinning.
There is a delicate balance between having a hard disk spinning down and up too frequently and leaving it spinning around the clock. If you have a filesystem that you want to have near instant access to but do so on an infrequent basis, you might like to use spindown to automatically spin down the disk containing that filesystem after you have finished accessing the drive.