I haven't seen the card before. Here's a guess as to use (only a guess) --
This may have been a check-cashing I.D. card, since the person's name & address & photo is personalized on the laminated card.
The issue date is before slot cards, but I've seen other check cashing cards (from other casinos) similar to this.

The Million Dollar Free-for-All may have been a promotion in connection with the 5 numbers printed on the card, like a 5-digit lottery ticket.
Come in and cash your paycheck and you get to draw or spin numbers and try to hit all 5 of your pre-printed numbers.

If they used numbers from 1 to 80 (similar to bingo or keno) then the odds of hitting 5 out of 5, in any order is about 24 million to 1. (80! / 5!)
With a payout of $1 million, the expected value of each play is about 4 cents ($1 million / 24 million). So ... if you come in and cash your paycheck, as long as you leave at least 4 cents behind, the promotion pays for itself.