Even the casing of a floppy drive?
If you look closely you'll see that the two standoffs on the rear of the floppy go to two large pads on the mobo that are connected to ground. Electronics works way better if everything is severely grounded.
While we're at it: the thin metal shield around the mobo provides very good grounding. As has been mentioned, the drive may provide no reliable ground for an alligator clip due to paint/oxydization even though it's grounded.
I usually keep the wall socket plugged in but switch off the mains switch. That way grounding is intact while no power whatsoever is to be found. While working on the machine I stay in contact or very frequently touch metal parts. Also, not shuffling your feet and handling delicate parts by the edges only ensures that your stuff runs reliable for some time to come.
