You bring in cash and play the machines and your winnings are recorded on the card. When you leave you bring the card to the cashier and they pay you and issue a 1099 for anything over $600.
Perfect way to launder money. Take $1M in dirty money, bet it all in Vegas and “win” $400,000. Now you’ve got a 1099 form to make the money “legitimate”. The $600,000 loss is the casino’s win.
Which is why they are more than happy to comp you all that free stuff.