A lorry turns up at my site and I take out a batch of items on which there is guaranteed to be valuable data.
Lorry operator takes each item and slips it into a slot.
Out of another slot falls bits of ground up waste.
How do _I_ _KNOW_ that what is falling out is actually what I put in? My disks etc contain valuable / sensitive / secret data otherwise I wouldn’t be paying for a bloke / blokess to bring their lorry round to destroy it all for me. So how do I know that this bloke / blokess hasn’t rigged their rig to provide the correct weight of scrap and hold onto my disks keeping them nice and safe for later data retrieval?