Hard disks do too but the difference is that for disks it's more or less an essential function.

If you don't leave plenty of spare sectors over the very inevitable bad sectors will cause the drive to fail outright.

Perhaps that's a decision I'd like to make myself but it's still an essential function no matter what.

Leaving spare capacity in my battery isn't an essential function. That extra hundred or two miles the first few years of ownership has value and if I use it right away I don't lose anything (except marginally faster wear on the battery pack)

The disk issue is about essential function. If I left no spare room the disk would have unrecoverable failures in days or weeks.

The battery issue is only about perception. If I left no spare battery charge the battery would function exactly the same. It's life would only be reduced in that I'd be using it more (this is assuming that my above theory is correct)