Casinos played a crucial role in making poker a mainstream game by providing a regulated environment, attracting a diverse player base, and increasing its legitimacy. By hosting high-stakes tournaments and televised events, they turned poker into a spectator sport and entertainment spectacle. Their investment in promotions, sponsorships, and player incentives helped grow its popularity, while structured rules and security measures ensured fairness, making the game more appealing to both casual and professional players.