But surely most bikes now have vertical dropouts? In which case the wheel won't move however loose the skewer is?
Well, almost vertical but the point is that the axle stub eats the inside of the dropout as well, effectively widening the slot.
Remember, that a bicycle is essentially a truss, with a potential space-frame at the back. I say potential, because for it to be a spaceframe, the axle has to be rigidly attached, which it isn't with poor QRs. Therefore it moves and frets and deteriorates. Add grit and sludge in the mix and you soon have an ex-frame.