Unused cardbacks were sent to customers that ordered from mail-order catalogs and they served as a Proof of Purchase so catalog customers could still participate in the promotions that required sending in Proofs of Purchase. They came without the figure in the bubble, instead they came with a figure in a baggie, often stapled to the cardback like the Lex Luthor shown below.
Note: These are NOT pre-production, and these are NOT proof cards! They are sometimes confused with proof cards since they have no bubble attached. They are categorized as Pre-Production on the Wiki since they fit in better with this group of items and because many people would look for them here since they are often misrepresented as such. See Also: Proof Cards
Below is a gallery of unused cardbacks.