The payer issues the form depending on the type of game you played, the amount of your winnings and other factors. You'll also get the form if the payer withholds taxes from what you won. You must report all your gambling winnings as income. This is true even if you don't receive a Form W-2G.
The odds for each game are stacked in favor of the casino. This means that, the more you play, the more the math works against you, and the better the chances are of you walking out of the casino with less money in your wallet than when you came in.