[dan000892]

Flash devices maximize the longevity of the cells by wear leveling. The on-board controller maintains a map between the physical cell layout and the logical layout that's changing with every single write operation.

Extra cells are used both reactively and proactively.

Reactively: Error when erasing or write/verifying a cell? Copy contents to spare cell, flag old cell as bad, update the map, maybe complain if running low on spare cells but otherwise call it a day.

Proactively: Cells are only rated for so many erase operations which directly informs the longevity (total bytes written) of a drive. MLC cells can be written to 3,000 times and SLC 20,000 but the latter is much more expensive. For use cases where greater longevity is required (without spending SLC money), why not put a disk together that has twice as many cells as it needs and wear level across all of them? If you `dd if=/dev/zero of=/dev/sdX bs=64k status=progress` your 32GB drive and it'll complete after 32GB but you have no way of knowing how many more cells that drive has that you didn't touch.

(Also as one of the parents pointed out, disk controllers are getting smart. You don't know if it was transparently compressing data too causing you to touch even fewer cells. ATA Secure Erase is the way to go.)