It's the nice perks that casinos give you just for playing. And by playing blackjack you can get the most in comps compared to other casino game. First let's make sure you understand how casinos determine how much your action is worth. Action is defined as the amount of money you risk over time.
Casinos use the following formula to calculate how much of a player's action they expect to win.
Now here is how a blackjack player can get more in comps then theoretical loss. Not a bad deal. You can even do better than this. If you play in very favorable games it's possible to reduce the casinos edge to virtually 0 and by learning card counting you can actually gain the advantage. Your theoretical loss in this scenario is 0 and the comps you earn will be gravy.