Because some of these disks were proclaimed worn out and not to be used. I still use all of them in 3 custom NAS builds. I sold 2 of them, the owners still haven't reported a disk failiure, that was 2 years ago. I use one of the NASes as my personal storage, mdadm in RAID5, I still haven't had a single disk fail on me. They were all full of "bad sectors" (logical, because of the bad contact between the head/preamp and the control board, bad data was being written to them, passed them with DRevitalize, all of the bad sectors were "reparied"), and yet, somehow, they still work.

Not to mention the numerous primary (OS) drives I've done this operation through the years and most of them still work fine, even though they have fulfilled their purpose (with the advent of SSD and all that). I've also compared the life cycle of identical drives that didn't get this treatment and ones that did. Most of the ones that didn't get this treatment are dead now (head crash in most cases).

Do this surgery to all of your drives as soon as you buy them (or at least after they're out of warranty), disable AAM/APM (wdidle3 in case of WD) (you can do this even if in warranty, it's a software/firmware tweak) and the disk will practically last forever.