Elastic memory allows the cushioning effect to occur as often as necessary. Sudden blows to this packing layer are distributed throughout, and little of the force is transferred to the packed item. The cushion material is usually a foam or rubber composition. Vibration can be simply defined as small impacts, repeated at intervals and over an extended period. Vibration may seem innocent because of the small size of force involved, but it can literally shake the paint off a surface.
Small cracks can spread due to prolonged vibration.
Packing materials such as foams can do double duty as both shock and vibration isolators. The hard case provides protection in the event of rough handling, which we must assume might occur. But the hard outer layer also allows even delicate items to be closely placed or stacked. And, as a practical matter, someone other than a museum conservator can safely move the object. Without this shell, we wouldn't dream of doing these things.