Packs were used for promotional deals years ago, different packs had different items in them. Once unpacked they turn into the items, but can't be repacked again. They also bypass QI when packed so for example anyone could have a demon pack sent to them, they could then unpack it and use the items.
Keep in mind that they lose their value the moment it's unpacked because they're considered rare.
A set is merely an item that someone has placed other items inside. Normal QI restrictions apply.