There were two cards.
One was the operators card... basically it put the game into the test menu when you swiped it... you needed it to register the cabinet. so you would swipe the card (or I think you actually left the card in the reader) and it would dial into IT with your info to register the cabinet at whatever location you typed in. The purpose for all this was if the game went online, IT billed the operator for all the online stuff (like if players bought cards, or played in tournaments, etc... part of the money was for IT, but the operator collected it out of the machine. IT needed to know the ops info so they could bill that money back from the op).
The second card was avaialable to the players. If you paid $3.00 extra, you could put in your info and IT would mail you a card. When you walked up to a machine and used your card, it allowed you to play in the online tournaments with your stats and info (address to mail the check to if you won!).