I play blackjack for a living. Maybe that is not a game you play, I have no idea. But depending on the exact rules and decks of the game, blackjack is about a .5% house advantage for a basic strategy player. (or a 99.5% return).

But with card counting that flips to close to a 1% player advantage (101% return). That little bit of a difference between 99.5% return (-EV) and 100.75 to 101% return (+EV) is the difference between losing and the 80K or so I have averaged a year in winnings for 13 years now in Vegas. I mean it is millions of dollars a year wagered to get that 80k or so, but that is all from that small difference between slightly -EV and slightly +EV. A difference of just a bit more than 1%.