Some people want to buy the ROM or at least have one with the purchase of the card so they can put the ROM on a flashcard.
This is useful for cases where they buy a PCE card, but only have a USA Express and want to take it somewhere.
If you buy the ROM and later go to buy the card, the ROM purchase price is removed from how much you owe for the card.
If you buy the ROM and never buy the card, you're probably an idiot.
These things glow in the dark.